UNIVERSITY OF CALIFORNIA
COLLEGE OF AGRICULTURE
AGRICULTURAL EXPERIMENT STATION
BERKELEY, CALIFORNIA
IRRIGATION BY
SPRINKLING
J. E. CHRISTIANSEN
BULLETIN 670
October, 1942
UNIVERSITY OF CALIFORNIA
BERKELEY, CALIFORNIA
Digitized by the Internet Archive
in 2012 with funding from
University of California, Davis Libraries
http://www.archive.org/details/irrigationbyspri670chri
CONTENTS
PAGE
Introduction 3
The use of sprinkler systems for
irrigating agricultural crops 3
General characteristics and classi-
fication of sprinkler systems. 5
Stationary sprinkler systems 7
Kotating sprinkler systems 7
Arrangement and spacing of
sprinklers 8
Layout of pipe lines 9
Systems with fixed sprinkler heads 10
Nozzle lines 11
Semiportable sprinkler systems .... 14
Portable sprinkler systems 15
Portable sprinkler pipe 16
Arrangement of portable sprinkler
lines and methods of opera-
tion 17
Use of field ditches 17
Use of pressure supply line ... 20
Drag-type sprinkler systems .". 21
Sprinklers for portable systems . . 22
Portable pumping plants 23
Pumps and accessories 24
Field ditches 26
Systems using stationary pumps
and pressue supply lines. ... 26
Stationary pumping plants ... 26
Pressure supply lines 27
Hydrants 30
Low-pressure systems for field
crops 30
Perforated pipe systems 31
Traveling sprinkler machines ... 32
Under-tree sprinkler systems for
orchards 33
Sprinklers for under-tree sys-
tems 33
Portable sprinkler pipe for
under-tree systems 35
Under-tree systems using sprin-
klers attached to hose 37
Portable drag -type sprinkler
systems for orchards 38
Sprinkling compared with other
methods of irrigation 41
Advantages of sprinkling .... 41
Conditions favorable to sprin-
kling ' 43
Limitations of sprinkling 44
Cost of sprinkling 45
Depreciation and interest on in-
vestment 46
Cost of operation 46
Explanation of table 2 50
PAGE
Hydraulics of sprinkler systems ... 51
Discharge from nozzles 52
Coefficients of discharge of noz-
zles and sprinklers 52
Discharge of small nozzles for
nozzle lines 54
Pressure required for sprinkler
operation 55
Flow of water in pipe 56
Formulas for friction loss in
pipe lines 57
Friction loss in standard pipe. . 60
Friction loss in welded steel
pipe 61
Friction loss in copper tubing. . 62
Friction loss in garden hose and
hydrants 63
Flow of water in pipes with mul-
tiple outlets 64
Derivation of equations for cal-
culating friction losses 64
Friction loss in sprinkler lines . . 67
Friction loss in nozzle lines ... 70
Discharge, pressure, and power
requirement relations for
sprinkler lines 71
Distribution of water from rotating
sprinklers 75
Sprinkler tests at Davis 77
Typical distribution patterns
for favorable conditions .... 78
Effect of low pressure on dis-
tribution from sprinklers ... 80
Effect of wind on distribution
patterns 84
Effect of high speed of rotation
on distribution patterns .... 86
Effect of variation in rate of
rotation on distribution pat-
terns 89
Desirable types of distribution
patterns and proper spacing
of sprinklers 93
A method of analyzing sprinkler
tests for uniformity of dis-
tribution 94
Distribution for geometrical
patterns 95
Desirable patterns for square
and equilateral-triangle ar-
rangements of sprinklers ... 97
Uniformity of distribution and
effect of spacing on actual
sprinkler patterns 103
CONTENTS — Continued
PAGE PAGE
Evaporation losses 310 Interception and subsequent
Evaporation from the spray 110 evaporation of water from
Direct measurement of evapora- plants 115
tion losses 110
spray 112
Evaporation losses from wet sur-
faces 114
Design and operation of sprinkler
Indirect method of estimating systems 116
?7!?_?rati0n l0SS fr°m the , -, n Methods of" operation ........ 116
Capacity of sprinkler systems. . 117
Eate of application 119
Evaporation losses' from* moist " DePth of application 119
soils m 214 Sprinkler capacity 122
Eate of evaporation from free Size of pipe 123
water surfaces 115 Acknowledgments 124
LIST OF TABLES
PAGE
-Sprinkler discharge required for various applications of water in
a twelve-hour period 33
-Eesults of field study to determine cost of operating portable sprin-
kler systems in Sacramento Valley 48
-Theoretical discharge of sprinkler nozzles 53
-Discharge of small nozzles for use on nozzle lines 55
-Friction loss in standard wrought -iron or steel pipe, for C = 100 ... 58
-Friction loss in standard wrought-iron or steel pipe, for C = 120. ... 59
-Friction loss in type M copper tubing 62
-Comparison of inside diameters and friction losses in copper tubing,
types M, L, and K 63
-Approximate friction loss in garden hose and garden hydrants .... 64
-Values of the factor F by which the friction loss in pipe must be
multiplied to obtain the actual loss in a line with multiple outlets 66
-Uniformity coefficients for various arrangements and spacings of
geometrical patterns B and E 97
-Uniformity coefficients for patterns G, I, and K 99
-Uniformity coefficients for patterns M, O, Q, and S 101
-Summary of sprinkler tests giving pertinent data together with the
calculated uniformity coefficients for different spacings between
sprinkler lines 104
-Summary of data in table 14 giving average uniformity coefficients
for several tests on the same sprinkler under similar conditions. . . . 108
-Uniformity coefficients for actual sprinkler patterns 109
Table 1,
Table 2,
Table 3,
Table 4,
Table 5.
Table 6,
Table 7,
Table 8.
Table 9,
Table 10,
Table 11.
Table 12.
Table 13.
Table 14.
Table 15.
Table 16.
IRRIGATION BY SPRINKLING
J. E. CHRISTIANSEN4
INTRODUCTION
This bulletin will discuss the application of irrigation water by sprin-
kling and will present the results of several years' research, together
with general information. Although intended especially for farmers who
are now operating sprinkler systems or contemplating irrigation by this
method, the bulletin includes some technical material essential to an
economical design of sprinkler systems — material of interest primarily
to engineers, irrigation contractors, and others engaged in manufactur-
ing, selling, and installing sprinkler equipment. The more technical
aspects of the subject, and most of the experimental results, appear in
the latter part of the bulletin. A closing section presents in nontechnical
form a discussion on design and operation of sprinkler systems.
THE USE OF SPRINKLER SYSTEMS FOR IRRIGATING
AGRICULTURAL CROPS
Sprinkling as a method of irrigation has been practiced in California
and elsewhere for about forty years. Before 1920 it was limited primarily
to truck crops, nurseries, and small fruits, and was practiced mainly as
supplemental irrigation in the more humid regions. Stationary overhead
sprinkler systems were first used in citrus orchards in some sections of
California about 1920. Most of such systems were installed, however,
between 1924 and 1928 in areas where surface irrigation was not entirely
satisfactory because of unusually pervious soil or because of topographic
features. Along the foothills east of Pasadena, the soil is gravelly in many
places and the slopes are rather steep for furrow irrigation, except
where orchards are laid out on contour grades; overhead sprinkling of
orchards had its inception in these areas.
Many of these systems were installed because of the extravagant claims
made by those interested in manufacturing or selling the equipment.
Sprinkling was supposed to save large quantities of water, to control
insects, frosts, and diseases, and to increase production. Some of these
early attempts proved unsatisfactory because of poor equipment and
1 Received for publication September 26, 1941.
2 This bulletin supersedes Extension Circular 4, Irrigation oy Overhead Sprinkling,
by H. A. Wadsworth, published in 1926.
3 Some financial assistance in the preparation of this bulletin was furnished by the
California Committee on the Eelation of Electricity to Agriculture.
4 Assistant Irrigation Engineer in the Experiment Station; resigned April 1, 1942.
[3]
4 University of California — Experiment Station
too wide spacing of sprinklers. After trial, a few of the systems were
abandoned, and furrow irrigation was resumed. As was gradually recog-
nized, sprinkling is not a cure-all; good results can be obtained only
when systems are properly designed and installed, with the right equip-
ment. Most of these original overhead systems represented investments
of $300 to $500 per acre or more. Since the drop in citrus prices about
1930, relatively few stationary sprinkler systems have been installed.
In 1932 the farm advisor's office in Los Angeles County made a sur-
vey6 of overhead irrigation of orchards in that area. One hundred grow-
ers, representing 1,504 acres, were questioned. The majority were,
apparently, well satisfied with their overhead systems. They had ex-
perienced very little trouble with the equipment and believed that the
change to sprinkling had increased production. Some had had trouble
with sprinklers stopping ; others with packing of their soil, runoff, and
the like. Only two reported abandoning the system. The survey also
indicated that the use of water was practically the same as for furrow
irrigation.
Lightweight steel pipe with quick-couplings for portable sprinkler
systems was first used about 1930. Its introduction completely changed
the picture of sprinkling as a method of irrigation. Whereas with sta-
tionary systems, the main consideration was the initial outlay, with port-
able systems for field crops the equipment costs sometimes as little as
$10 to $15 per acre and usually under $50. The expense of operation be-
came the important item ; this, annually, sometimes exceeded the invest-
ment in equipment. These portable sprinkler systems were better adapted
to, and were used more on field and truck crops than in orchards. Ac-
cording to a survey made in 1936, a greater acreage of sugar beets was
being sprinkled than any other crop. Sprinklers were being used on
nearly all field and truck crops as well as in orchards.
Most of these systems are used in areas not adapted to surface irriga-
tion. Near Clarksburg on the Sacramento River south of Sacramento,
subirrigation had been practiced almost exclusively. In 1931, probably
because a series of dry winters resulted in insufficient natural leaching,
alkali spots were beginning to appear in many places ; they were espe-
cially noticeable in fields planted to beans. To counteract the upward
movement of the soluble salts, portable sprinkler systems were tried.
Within a few years more than a hundred systems were in use in the
Sacramento Delta, farther north in Sutter Basin, and near Meridian.
Because of the high water table in these areas, the irrigation requirement
was low compared with that in other places of similar climate. Sprinkler
5 Salter, A. G. Orchard overhead irrigation. Pacific Eural Press 131(11) : 346-47.
Mar. 14, 1936.
Bul. 670] Irrigation by Sprinkling 5
systems could therefore cover relatively larger areas at lower per-acre
costs. Since surface irrigation was not feasible in many places, compari-
sons were most frequently made between sprinkling and subirrigation;
in general, results were better with sprinkling.
Sprinkling has also become very popular for irrigating certain truck
crops near Moss Landing in the Salinas Valley, where both the type of
soil and topography have been reasons for changing from surface meth-
ods. Another factor that makes sprinkling economical in the coastal areas
is the lower water requirement due to low temperatures and high humidi-
ties.
Portable sprinkler systems have also been installed in many scattered
areas for irrigating pastures and field crops. They have been used rather
extensively in Oregon, especially for pasture irrigation. Many were in-
stalled in areas where previously irrigation had not been practiced.
About the same time that portable sprinkler pipe was introduced,
under-tree orchard sprinkler systems also came into use. These systems
consist of small sprinklers, mounted on short risers, which cover the
areas between adjacent trees. Because stationary equipment of this type
interferes with cultivation and with other orchard practices, most of
these systems are portable. The simplest type of portable unit consists of
several small sprinklers on low stands joined with %-inch garden hose.
This portable unit is supplied from small garden hydrants on buried
supply lines. Later a drag-type sprinkler system using lightweight steel
tubing joined with special couplings was developed. The introduction of
portable sprinkler pipe in small sizes led to the use of this pipe for
under-tree orchard systems. The last few years have seen rapid advances
in the development of under-tree equipment. Orchard sprinkling with
such systems is practiced mainly in San Diego County, in the La Habra
Heights section of Los Angeles County, and in some parts of Santa
Barbara and Ventura counties.
GENERAL CHARACTERISTICS AND CLASSIFICATION
OF SPRINKLER SYSTEMS
All sprinkler systems have certain features in common. They consist
essentially of pipes with sprinklers or nozzles for distributing the water
over the area to be irrigated. To accomplish this, the water must be dis-
charged at a high velocity, which is obtained by operating the system
under pressure. This pressure may be provided by gravity from a water
source at a higher elevation or by pumping.
Irrigation by sprinkling differs from other methods in that the water
is distributed over the soil by mechanical means, whereas for other
methods of irrigation the water is finally distributed by the soil itself.
6 University of California — Experiment Station
The uniformity of application by sprinkling depends primarily upon
the ability of the system to apply equal amounts to all parts of the area.
For surface irrigation the uniformity depends upon the surface condi-
tion, and upon uniform permeability of the soil as well as the ability of
the system to distribute the water uniformly to local areas. Although
sprinkling is frequently called an imitation of rain, it differs from rain
in many respects. The distribution is never so uniform as with rain. The
rate of application and amount applied are largely under control, a
feature that overcomes some of the detrimental effects of rainfall. The
effect on plants may be different, especially in connection with the spread
of disease. Since sprinkling is commonly carried on during warm, sunny
days it has little effect on the humidity except during the application,
whereas rain is generally associated with cloudy weather and high hu-
midity lasting a considerable time.
Despite certain common characteristics, sprinkler systems have many
differences. They may be classified according to their portability, or ac-
cording to certain mechanical features. They are called "portable'* when
most of the mechanical equipment can be readily moved from place to
place over the area irrigated; "semiportable" when only a minor part
of the equipment is moved, such as the risers and sprinkler heads ; and
"stationary" or "permanent" when all the equipment is fixed. Obviously,
definite lines cannot always be drawn between these three groups, espe-
cially between portable and semiportable systems. This classification
overlaps completely on any grouping according to mechanical features.
Sprinkling is frequently called "overhead irrigation." This is some-
what confusing because orchard systems are often classified as "over-
head" or "under-tree." The overhead systems distribute the water over
the tops of the trees, whereas under-tree systems distribute it near the
ground between and under the overhanging branches of the trees. Under-
tree sprinklers are sometimes called "low-head," or "ground" sprinklers.
Either of these systems may be portable or stationary. Because stationary
under-tree sprinklers interfere with cultural practices in an orchard
and are also more expensive, most under-tree systems are portable. Al-
though many overhead systems are portable, the high risers make
them somewhat difficult to move. The pipe lines of stationary overhead
systems are generally placed underground and the sprinklers directly
over the trees, completely out of the way of other orchard operations.
Sprinkler systems may also be classified according to method of dis-
tributing the water, whether by rotating sprinklers, by fixed heads, or
by nozzles along a pipe line which is rotated back and forth through an
angle of about 90 degrees to cover a strip on each side of the line. This
latter type is called a nozzle line.
Bul. 670] Irrigation by Sprinkling 7
STATIONARY SPRINKLER SYSTEMS
Stationary systems are used to a considerable extent in California.
Most of the original orchard systems were of this type, with rotating
sprinklers mounted on high risers over the trees. Other kinds of sta-
tionary systems include those with fixed sprinkler heads, used exten-
sively for lawns and ornamental plantings and to a limited extent for
orchards ; and nozzle lines used principally for truck crops, nurseries,
and other special purposes.
ROTATING SPRINKLER SYSTEMS
Rotating sprinklers have capacities of 1 to more than 100 gallons per
minute. Some of the larger sprinklers are designed for pressures of 60
to 100 pounds per square inch, and will cover circles up to approximately
200 feet in diameter, whereas some of the small ones will operate on
pressures of 10 pounds per square inch or less. In general, the larger
the nozzles, the higher the pressure required for best performance. Some
very large sprinklers, with capacities of several hundred gallons per
minute, have been used for irrigating bananas in Central and South
America, and for other crops in certain European countries. Because
of the extremely high pressures required for their operation, these large
sprinklers are not considered economical in California. For supplemental
irrigation, where the annual water requirement is low, high pressures
and large sprinklers may be more economical than small sprinklers be-
cause of the greater permissible spacing of pipe lines.
Rotating sprinklers are of two types : whirling sprinklers, that rotate
rapidly, and slow-revolving sprinklers, that rotate slowly. Slow-revolv-
ing sprinklers ordinarily rotate at a speed of one or two revolutions per
minute ; some much slower. The slower a sprinkler rotates, the larger the
area it will cover. When operating under pressures of about 40 pounds
per square inch, slow-revolving sprinklers will cover areas up to about
120 feet in diameter. Whirling sprinklers of similar capacities, operat-
ing under equal pressures, will cover areas 50 to 70 feet in diameter.
The slow-revolving sprinklers are therefore more desirable, since they
permit a greater spacing of pipe lines.
Slow rotation minimizes wear, and thereby prolongs the life of the
sprinkler. Various types of driving mechanisms are used to produce a
slow rotation and prevent the sprinklers from stopping. Sometimes slow-
revolving sprinklers vary greatly in their speed and this variation may
appreciably affect the distribution of water. Whirling sprinklers tend
to rotate at a more constant rate because of the momentum of the re-
volving parts. They are generally less expensive than slow-revolving
8
University of California — Experiment Station
sprinklers. Figure 1 shows a few of the different types of rotating
sprinklers.
Arrangement and Spacing of Sprinklers.— Though absolute uniform-
ity is not essential, large variations in depth of application are not
desirable, particularly with stationary systems where the cumulative
effect of either excessive or inadequate amounts becomes serious. The
distribution of water from sprinklers depends upon several factors.
Careful study, however, indicates that approximately uniform applica-
Fig. 1. — Different types of rotating sprinklers. Types with external driving
mechanisms: A, spinning wheel type; B, D, and J, oscillating spring types;
C and E, pendulum types. Keaction-drive sprinklers: F, E, and I, whirling
sprinklers ; G and K have vibrators for slow rotation. Internal-drive sprinklers :
L, friction-drive type, and M, gear-drive type.
tion is possible with sprinklers covering circular areas. The application
can be uniform only when (1) the type of pattern produced is correct
for the arrangement of sprinklers, (2) the sprinklers are correctly
spaced, (3) the sprinklers rotate at a uniform rate, and (4) there is no
appreciable wind.
For stationary installations, sprinklers are arranged in squares, tri-
angles, or rectangles with a greater spacing between the pipe lines than
between sprinklers on the line. For stationary orchard systems, the
sprinklers are usually mounted over certain trees. With trees planted on
a square, an equilateral-triangle arrangement of sprinklers is not prac-
ticable. All sprinkler spacings must be multiples of tree spacings, which
generally vary from 18 to 24 feet. The maximum practical spacing for
medium-sized slow-revolving sprinklers operating under adequate pres-
Bul. 670]
Irrigation by Sprinkling
sure is a sprinkler every fourth tree, in every fourth row. For this spac-
ing, a triangular or staggered arrangement will probably give better
distribution than a square. For most sprinklers, a spacing of three tree
rows in both directions would be better, in which case staggering is not
Sprinklers
o
Mafn
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Lateral
o—
a. - Square
b. -Spacing between fines twice spacing a fang iine
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0 0 <
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7
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d- - Eaa/faferaf frfang/e
Three raws of sprinft/ers on ane fateraf
Fig. 2. — Some of the various arrangements of sprinklers and
pipe lines for sprinkler systems. The letters St and S2 denote spac-
ing between sprinklers, and spacing between pipe lines, respec-
tively.
practicable. Other possibilities include a sprinkler in every other tree in
every third or every fourth row with the sprinklers staggered, or ar-
ranged in a rectangle. According to analyses of both actual and geo-
metrical patterns (discussed in a later section) with fairly close spacings
along the line there is little to be gained from staggering. Square and
rectangular arrangements of sprinklers generally permit better dis-
tribution around the borders of the field than triangular or staggered
arrangements.
10
University of California — Experiment Station
Layout of Pipe Lines. — The usual arrangement of pipe lines for sta-
tionary sprinkler systems consists of a central main with parallel laterals
at right angles to the main line, carrying the sprinklers. Each lateral is
provided with a shutoff valve so that it can be operated independently.
Since stationary systems are generally confined to relatively small areas
such as 10-acre tracts, these laterals will seldom carry more than six or
eight sprinklers. Larger areas are sometimes divided into smaller sec-
tions treated separately. In all cases a main supply line through the
center of the area is most desirable. Where the water comes from a well,
the cheapest arrangement is to place the well approximately in the
center of the tract to be irrigated ; this minimizes the pipe sizes required.
Figure 2 shows various arrangements of sprinklers and pipe lines.
Fig. 3. — Different types of fixed heads. The center three are lawn
heads and the two end ones are commonly called shrubbery heads.
Both are used for under-tree orchard systems.
For an equilateral-triangle arrangement of sprinklers, about 9 per
cent in length of pipe required will be saved if three rows of sprinklers
are supplied from one lateral (arrangement d compared with c, fig. 2) . A
similar arrangement can be used for sprinklers placed in a square. Al-
though, in this case, there is no saving in total length of pipe required,
and the main laterals have to be increased in size to carry three times
as much water, the cost may be less, since the sublaterals can be smaller.
For example, a 3-inch main lateral with 1-inch sublaterals might replace
three 2-inch lines.
SYSTEMS WITH FIXED SPRINKLER HEADS
Fixed heads are used mainly for lawns and for portable under-tree
systems for orchards. These heads have no moving parts; they cover
areas 15 to 25 feet in diameter with a fine spray fairly evenly distributed.
Although most fixed heads cover circular areas, some are designed to
cover square areas. Figure 3 shows a few fixed heads of different types.
The use of fixed heads on field and orchard systems is limited. For
stationary systems, the cost is excessive, because of the close spacing
required, and furthermore, the closely spaced heads are objectionable
from the standpoint of cultivation. The minimum rate at which water
Bul. 670]
Irrigation by Sprinkling
11
can be applied with these sprinklers is often too high; the soil cannot
absorb it rapidly enough to prevent accumulation of water in low places
or runoff. Normal rates of application with these heads range from about
0.7 inch to 3 inches per hour, whereas with rotating sprinklers rates as
low as 0.1 inch per hour are possible, and rates of 0.2 to 0.5 inch per hour
are usual. Many bare soils do not absorb water rapidly enough to permit
the use of fixed heads, except when only light applications are made.
sWozz/e3
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vo/ye
H
\\v
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V
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>\:-°
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Fig. 4. — Detail of typical nozzle line assembly for hand rotation. The special fit-
tings required can be secured from dealers handling sprinkler equipment. (From Ext.
Cir. 4.)
On the other hand, simplicity, absence of moving parts, and low cost
make fixed sprinkler heads highly desirable where conditions permit
their use, such as for portable under-tree orchard systems on permeable
soils.
NOZZLE LINES
Nozzle lines consist essentially of parallel lines of pipe (% to 1%
inches in size) equipped with small brass nozzles, usually spaced 2 to 4
feet apart (fig. 4). Being relatively expensive, they are generally used
only for crops yielding a high gross return, such as certain truck crops
and small fruits, and for nurseries, greenhouses, lawns, and other special
purposes. They are occasionally used in orchards, especially on terraced
plantings.
12
University of California — Experiment Station
The pipe is generally supported 4 to 7 feet above the ground on posts
spaced about 15 feet apart. Where it is not necessary to provide passage
underneath for cross-cultivation, lines about 4 feet above the ground
permit convenient access to the nozzles for servicing. Occasionally nozzle
lines are supported by a suspension cable from much higher poles 100
to 200 feet apart, an arrangement that lessens the obstruction to cultiva-
tion. To cover a strip of ground on both sides of the lines they are oscil-
lated through an angle of about 90 degrees, generally by means of a
water-operated oscillating motor. Figure 5 illustrates this type of system.
Fig. 5. — Nozzle line used for irrigating strawberries. This line is rotated
back and forth by a hydraulic oscillating motor at the head end of the line.
(Six nozzles, of a special type, shown in the foreground, were being tested for
distribution of water; this accounts for the difference in appearance of the
jets.)
A complete line of equipment is available for nozzle lines, including a
variety of nozzles, ranging from those that throw a round jet for maxi-
mum coverage, to others that have deflectors to break up the jet for
narrower strips. Some nozzles, having orifices of triangular shape, are
designed to distribute the water over a considerable area without oscil-
lating the line. Special roller saddles on top of the posts carry the pipe
and reduce to a minimum the friction in rotating the line. These fittings
are available for either wood or pipe posts. Special hand-turning unions
are also available. For best performance, however, nozzle lines should
be equipped with oscillators that slowly rotate the line, resulting in a
fairly uniform distribution of water when the lines are correctly spaced.
These oscillators are usually double-acting piston devices that operate
Bul. 670] Irrigation by Sprinkling 13
from the water pressure. Being fairly expensive, they are sometimes
moved from one line to another, one oscillator being used for several
lines. Sometimes one hydraulic oscillator is used to rotate a number of
parallel lines by means of cables attached to arms on each line. For good
results the mechanics of such systems must be carefully worked out.
Because of the very small diameter of the nozzles used on these lines
they are easily clogged, especially when the water is from an open source
where algae might grow. Special screens are available for such condi-
tions. One make of equipment utilizes small individual screens on the
nozzles; but since these project into the pipe and increase the frictional
resistance, they are less desirable than properly designed screens at the
head of the line. Screens should have ample area in order to cause only
a small resistance to flow even when much of the total area is clogged.
They must also be designed to facilitate cleaning. If the source of water
is free from anything that would cause clogging, screens are unnecessary.
Flushing valves are used at the end of the line for flushing dirt and pipe
scale from the line. Such material always tends to settle out and to clog
nozzles only near the end of the line.
Special union couplings with squared sockets are available to facilitate
the lining up of the nozzles. They are especially important on portable
lines that are moved from one set of supporting posts to another.
Since the discharge of all nozzles varies with the pressure, the only
way to secure a uniform discharge along the line is to limit the pressure
loss to a small part of the normal pressure on the line. Frequently the
friction loss is appreciable ; the pressure along most of the line may be
very different from that at the head. Pressure-gauge connections with
shutoff cocks at each end of the line are desirable, for they permit the
pressures to be readily determined by means of a portable pressure
gauge. A permanently mounted pressure gauge is sometimes used at the
head end, but seldom are the pressures determined at other points. Fric-
tion loss in pipe lines is discussed in detail later.
Nozzle lines are usually operated at pressures of 25 to 40 pounds per
square inch. The width of strip effectively covered increases with in-
crease in pressure up to about 40 pounds per square inch. Above this
there is little or no increase in width because of a greater dispersion of
the jets. Where the pressure is adequate, the lines are spaced about 50
feet apart. Sufficient pressure should be provided to overcome friction
losses and to leave sufficient pressure for effective operation.
Nozzles of the type commonly used on nozzle lines have capacities
ranging from about 0.15 to 0.30 gallon per minute at a pressure of 30
pounds per square inch. Special nozzles, occasionally used, have capaci-
ties up to 0.70 gallon per minute. Friction loss in nozzle lines, being at
14
University of California — Experiment Station
any point proportional to the square of the velocity, will vary appreci-
ably with different combinations of nozzles and pipe sizes, and this must
be kept in mind when designing sprinkler systems.
Parta/>/e riser
Va/re. A/ot reaa/red w^ea
/at era/ /e snat off
for n?ov//?a sprinA/ers
l/l/fre t/e
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Stationary riser. Scmet/mes
a size /aryer /ban por/nt/e
riser /o increase sriffness
and decrease fric//ona/ /osses.
Ground ^
' /at era/. Son?et/mes /eft
on /op of yroaad and
s/a/fonary riser on?// ted
Fig. 6. — Details of riser for semiportable sprinkler sys-
tem using portable sprinklers on a stationary pipe distri-
bution system.
SEMIPORTABLE SPRINKLER SYSTEMS
Some sprinkler systems have stationary pipe lines and portable sprin-
klers that are moved from place to place. They are especially desirable
in orchards; they are less expensive than stationary systems and they
obviate certain practical difficulties encountered with portable systems,
such as holding high risers in vertical positions and in moving them from
place to place. Figure 6 illustrates a common arrangement. A short
stationary riser with valve extends 4 or 5 feet above the ground. The
upper part of the riser and sprinkler is portable, and only the number
Bul. 670] Irrigation by Sprinkling 15
operated at one time are required; thus a considerable saving in the
cost of sprinklers is effected. For example, 6 or 8 sprinklers may suffice
for a 10-acre tract, whereas a stationary system would require more than
100. In addition, there is an appreciable saving in the cost of pipe lines
because smaller sizes may be used when each sprinkler is operated on a
different lateral. For example l^-inch laterals may suffice, whereas
2-inch (or larger) laterals may be required where several sprinklers are
operated simultaneously on one lateral.
A somewhat similar type of semiportable system is extensively used
for parks, golf courses, athletic fields, and other large lawns. A sta-
tionary-pipe distribution system is installed with an automatic shutoff
valve at each sprinkler location. Sprinklers are attached to these valves
by simply inserting a special coupler and giving it a turn. These valves
are sold under such trade names as "super valves," "snap valves," and
"lawn valves." Such systems are also suitable for certain agricultural
crops, though the cost of the automatic valves makes them rather expen-
sive.
When overhead sprinkling first came into use, a popular type of
semiportable system utilized sprinklers mounted on a high riser on a
portable stand and attached to a hydrant with garden hose. This ar-
rangement permitted relatively wide spacing of laterals and hydrants,
since several sprinkler locations could be served by one hydrant. The
locations of the sprinklers can be varied for different irrigations to
effect a more uniform seasonal application for all places in the orchard.
The arrangement is not, however, entirely satisfactory because of the
difficulty of moving the sprinklers.
Another type of system used in orchards that might be classed as
semiportable consists of stationary pipe lines supplying water to port-
able under-tree sprinklers. These are discussed more fully in the section
on "Under-Tree Sprinkler Systems for Orchards."
PORTABLE SPRINKLER SYSTEMS
The type generally known as a portable sprinkler system originated
about 1930. Essentially it consists of a sprinkler line of special light-
weight portable pipe with quick-couplings, together with a pumping
plant. Sprinkler pipe comes in standard lengths of 20 feet ; other lengths
are furnished on special order. Sprinklers are usually spaced 20, 30, or
40 feet apart. The pipe is moved across the field by carrying one length
at a time. Several makes of systems are available, each with a different
kind of coupling, some of which are shown in figure 7. These systems
are used largely for irrigating field and truck crops, such as sugar beets,
peas, beans, and onions.
16
University of California — Experiment Station
Many systems operate with a portable pumping plant supplied from
a field ditch along one side or through the center of the field. The entire
system can be moved from one field to another. Where the water supply
is obtained from wells or where, because of the topography, ditches
through the field are not feasible, a stationary pumping plant and pres-
sure supply line are used.
PORTABLE SPRINKLER TYPE
When these systems first came into use only 4-inch pipe was avail-
able. Now some of the makes can be obtained in sizes of 1%, 2, 3, 4, 5, and
Fig. 7. — Some of the different types of portable sprinkler-pipe couplings:
A, Shur-Rane, ball-and-socket type; B, Montague; C, Calco (3-inch) ; D, Eain
Storm; E, Pierce; F, Calco (4-inch); G, Wilson; H, Shur-Rane; I, Calco
(4-inch, coupled).
6 inches and in any desired length. For field-crop systems, lengths of 30
and 40 feet are sometimes used. For orchard systems, pipe of the same
length as the tree spacing is desirable. The longer lengths are slightly
cheaper because they require fewer couplings.
Most of the portable sprinkler pipe is made from electric-welded steel
tubing, 16-gauge being used for 4-inch and larger pipe, and 18-gauge
for 3-inch and smaller sizes. Lighter-weight sprinkler pipe, made from
22- and 24-gauge metal, has come into use recently. This is somewhat
lighter to handle and is strong enough to withstand the pressure re-
quired; but it is less durable than the heavier pipe. Some of the couplings
(fig. 7) are attached permanently to the pipe, whereas others are separate
Bul. 670]
Irrigation by Sprinkling
17
and simply connect two lengths. In some, the sprinkler outlets are in the
couplings; in others, the outlet is welded to the pipe a short distance
from the coupling. The pipe is usually galvanized after fabrication.
1
*
♦
I
V
*
r'-l
I
•11
^ I
J'
It
Sprin/c/er
PorfaS/e spr//7k/er //he
Porta6/e pc/mping p/a/r£
•D/'ic/i
Fig. 8. — Single-line arrangement of portable sprinkler system
supplied from a ditch along one side of the field.
rv? ?
^ <j>-«~- Spr/nA/er
1!
-IV-
£*
\*-P{/fl7pf/7g p/tfflt
D/fc/7
I i
t i
♦ t
\ \
i i
A A
5pr//?kfer /f/?e
Fig. 9. — Split-line arrangement of portable sprinkler system sup-
plied from a ditch running through the center of the field. This results
in a more economical operation than with the single-line arrangement
shown in figure 8.
ARRANGEMENT OF PORTABLE SPRINKLER LINES AND
METHODS OF OPERATION
Use of Field Ditches. — There are several possible arrangements of
portable sprinkler lines. The most common are the single-line and split-
line arrangements (figs. 8 and 9). The term "split line" is a misnomer,
18 University of California — Experiment Station
but has become more or less established by usage. With the single line
the supply ditch is located along one side of the field, and the line of
portable pipe extends across the field. This arrangement is practical for
pipe lines not exceeding 1,000 feet in length. It is generally used where
an existing ditch, from which the water can be pumped, is located along
the field border. An example appears in figure 10. The sprinklers are
operated until the desired application is made ; then the pumping plant
is shut down while the pipe and pumping plant are moved to the next
position. The pipe is usually moved by two men carrying one length at
a time. Under ordinary conditions, pipe can be moved at an average
Fig. 10. — Portable sprinkler system irrigating beans in Sutter Basin; a
single line of 4-inch pipe with sprinklers 20 feet apart. The portable pumping
plant is at the far end of the line.
rate of about 30 lineal feet of pipe per minute. From 5 to 10 minutes are
required to move the pumping plant and start operation. Where only
light applications are needed and lines are operated in each position for
a relatively short period — two hours or less — a large part of the total
time is lost in moving. Another disadvantage of the single-line arrange-
ment is the high friction loss which is about eight times as great as with
split lines of the same size and total length and same capacity. Single
lines often necessitate the use of larger pipe, or smaller sprinklers, than
split lines.
The split-line arrangement is the most satisfactory for portable sprin-
kler systems using portable pumping plants. It permits the use of mini-
mum pipe sizes and allows for nearly continuous operation of the
pumping plant. The pipe line on one side of the pump is shut down and
moved to the next position while the line on the other side continues to
operate. The pump is then shut down, moved to the next position, con-
nected to the line that has been moved, and started up. The other line
is then moved, connected to the pump, and turned on. With the same
Bul. 670]
Irrigation by Sprinkling
19
throttle setting, and with only one of the two lines in operation, the pres-
sure is increased by about 50 per cent, and the discharge through the
one line is about 60 per cent of the two lines. The plant is therefore in
effective operation most of the time. The actual time lost by moving is
slightly less than the time required to move one of the lines. The operat-
ing efficiency is therefore considerably higher than for the single-line
arrangement; when feasible, the split-line arrangement should be used.
Sometimes two parallel lines are operated on the same side of the
supply ditch as shown in figure 11. When it is time to move, the front
B
'A \B' W
1 t i
2 k 2 k
i M
> \ \ y SprMer
& i
* k <b kje^ Poria6/e sprM/er //'/?e
i ^ |"
K
y ^fiortaS/e pumping p/e/ii
rD/'ich
Fig. 11. — Double-line arrangement of sprinklers illustrating how
two parallel sprinkler lines can be operated on the same side of the
supply ditch. This arrangement being undesirable should be used
only where other arrangements are not feasible.
line, A, is shut down and moved twice the spacing of the lines to A'. The
pumping plant is then brought up, and this line is placed in operation.
The rear line, B, is then moved to B', and put in use. This method pro-
vides for nearly continuous operation of the pumping plant but is ob-
jectionable because it necessitates carrying all the pipe twice as far and
moving the rear line across wet ground. It should be used only where
other arrangements are not practicable. Figure 12 shows a sprinkler
system using a double line.
Another method of operating, called the alternate-line arrangement,
is used where the pumping-plant capacity is not adequate for operating
two lines at a time. The arrangement of the portable lines is usually the
same as for the split-line arrangement. The method is similar except that
only one line is operated at a time. After the second line is moved, the
water is not turned on until time to turn off the first line. This arrange-
20 University of California — Experiment Station
ment permits almost continuous operation of the pumping plant at full
capacity, but requires twice as much pipe as a single line. Because of the
higher operating efficiency, a larger area can be served by a pumping
plant of a given capacity. This method is especially suitable where the
pipe is moved at frequent intervals.
Use of Pressure Supply Line. — Where water is supplied under pres-
sure, or where for other purposes a stationary pumping plant is more
desirable, water can be supplied to the portable sprinkler lines through
a pressure supply line. The ideal arrangement is shown in figure 13
illustrating a supply from a well located in the center of the field. With
Fig. 12. — Double-line 3-inch portable sprinkler pipe with sprinklers spaced
40 feet apart, being used to establish Ladino clover pasture.
the pumping plant at this position, minimum pipe sizes can be used, and
friction losses are less than where pumping plants are located at the
boundaries of the field. The water is supplied to the portable lines from
hydrants located along the pressure line. This arrangement, although
the most satisfactory from the standpoint of operation, is somewhat more
expensive than systems using portable pumping plants, since a stationary
pressure supply line represents an investment of $25 to $50 per acre —
usually more than the cost of the portable pipe. Any number of portable
laterals can be used on one pressure supply line. With two or more
laterals the pumping plant can be operated continuously. When several
laterals are used, they should be arranged so that the friction loss in
the supply line will be a minimum, and so that the laterals will not
interfere with each other. A large installation of this kind, with several
laterals, is used in the San Joaquin Valley for irrigating cotton. A crew
is kept busy moving pipe. This provides more effective use of the labor
and may therefore result in an appreciable saving.
Bul. 670] Irrigation by Sprinkling 21
Under some conditions a portable pressure supply line is more feasible
than a stationary one. This is especially true where a large sprinkler
system is used to irrigate several fields and the portable line can be
used effectively in several locations.
Portable pressure mains are usually not provided with valve outlets,
but are directly connected to the laterals. The main line is shortened
or extended by removing or adding sections of pipe to fit the various
locations of the laterals. When two or more laterals are supplied from
one main, portable valves are required, or the system must be shut down
?<??<???
*
> |* k k * t"
£$ 1 i l I i
> ^ k 4 \ h-
^Sz-+- S2-*- S2-+- Sj,-j- 5/-i
V > t t I
^4 Ǥt 4 * \
— ru, 1 l — £. — .
Pan?/.
4f
vng
p/a/it
<
Nc
Supp/y f//7e
ite. Wften two /atero/s ore
1
k
1
\
1
k
1
-V-
1
1
1
1
h
i
♦
i
i
r-
♦
1
♦
1
i
- 9"
1
1
>j 1
i!
1
usee/, jtort o/?e ot opposite
ere" of fte/d fro/7? ttie otter.
1
i>
l
A
i
a
1
^i
Fig. 13. — Arrangement of a portable sprinkler system operating
from a pressure supply line with a stationary pumping plant supplied
from a well.
while both laterals are moved. Although portable supply lines may offer
a saving in pipe cost, they are less convenient than stationary supply
lines.
Drag-Type Sprinkler Systems. — Since the main cost of operation of
portable sprinkler systems is for labor for moving portable sprinkler
pipe, some interest has been shown in drag-type systems, especially for
orchards, in which the portable pipe is moved by dragging endwise. The
power for moving the pipe may be supplied by a small tractor, a team,
or one horse. Small units are sometimes moved by hand. When one or
two lengths of pipe are used ahead of the first sprinkler on the line, it is
not necessary to work on wet ground to move the pipe.
Figure 14 illustrates a completely portable drag-type system with a
portable pumping plant supplied from a ditch through the center of the
field. With such systems the portable lateral is taken apart and moved
forward with the drag unit. A wheel cart at the head end of the unit is
22
University of California — Experiment Station
used both for holding the risers in a vertical position and for carrying
the lateral forward. A more satisfactory arrangement of the drag-type
system (fig. 15) illustrates the use of a stationary pressure main and
laterals with portable drag units. Relatively small pipe can be used for
the laterals when the drag units are distributed so that only one or two
are served by each lateral. Drag-type systems for orchards are further
discussed later.
SPRINKLERS FOR PORTABLE SYSTEMS
Slow-revolving sprinklers are most satisfactory for portable systems.
Because of continuous operation, sprinklers on portable systems receive
C x<*'Por£ab/e /at era/
3.\ „„ Mov/'ra a/7/'t ar-d
^.^.<>.<>.<>.<v.^4^.<>.<>.<83_l=1 ^ /atera/
A' I A" I
■0---0---0-0--- °" -0---0— jo--- 0-O—0---0- --»--■<>-■ ■©—■! r /)/ f /~h
Porta6/e pu/np/ng p/ant
t/n/ts /noi/ea' forward
w/ta tractor or tea/77.
Aa/'aceflt sect tons of /atera/
moi/ea w/tft dray aa/t.
Fig. 14. — Arrangement of a drag-type sprinkler system with a
portable pumping plant supplied from a ditch.
much harder service than those on stationary systems. Being subjected
to considerable abuse in moving operations they must, to be satisfactory,
be ruggedly built. When the portable systems first came into use, very
few of the sprinklers previously used on stationary systems proved suit-
able. Some wore out in less than a season. On some portable systems,
sprinklers may operate 2,000 hours or more during a single season,
whereas on stationary systems they seldom operate more than 100 hours
a season.
The length of portable line required is usually determined by the di-
mensions of the field, or fields, to be sprinkled ; the number of sprinklers
used, by the length of line and the sprinkler spacing. The capacity of the
pumping plant is limited by the source of power available. In many
cases, farm tractors, used primarily for other operations, are utilized.
Capacities of sprinklers range from approximately 5 to 25 gallons per
Bul. 670]
Irrigation by Sprinkling
23
minute, the choice depending upon a number of conditions. The capacity
of some sprinklers can readily be changed by varying the size of nozzles.
Sprinklers of the capacity that properly adapts the size of the pumping
plant to the length of line required should therefore be selected. When
the nozzles are too large, operating pressures are inadequate for proper
performance ; when they are too small, inefficient use is made of the
pumping unit. Efficiency necessitates operating the sprinklers at the
lowest pressure that gives satisfactory distribution of water, and using
the pumping plant to full capacity.
The sprinklers are mounted on short risers directly over the portable
lateral
fyc/rartts
Sta/iorfay /na//7
— 17
Pt//7?p//7tf p/a/7t
Fig. 15. — Arrangement of a drag-type sprinkler system with porta-
ble drag units supplied from a stationary pipe system. Solid lines
indicate initial position of portable units ; dotted lines, the position
after first move. Units E to L, shown in initial position, are moved
in similar manner.
pipe. These risers should be high enough to place the sprinkler over the
top of the crop grown so that nothing interferes with the rotation of the
sprinklers. For low-growing field crops such as sugar beets and peas, 12-
or 18-inch risers are adequate; for higher-growing crops such as cotton,
30- or 36-inch risers may be necessary. High risers are objectionable be-
cause they tend to tip over ; they also make it more difficult to move the
pipe. In some cases, short bases are clamped to the pipe to prevent tip-
ping. Some of the special couplings have footings to which strips of
wood can be attached when required.
PORTABLE PUMPING PLANTS
There are two types of portable pumping plants : the tractor type that
utilizes its own power both to operate the pump and to move from one
24
University of California — Experiment Station
location to another ; and the independent engine type, where a separate
engine is used for operating the pump. The latter may consist of a truck,
a trailer, or skids on which the pump and engine are mounted. The
tractor type is most common, probably because in many instances a
tractor required for other farm work is available during the irrigation
season. Figure 16 shows a typical plant. Tractors eliminate the necessity
of other power for moving the pump from one position to another. Be-
cause trucks tend to mire into mud along ditches, they are sometimes
objectionable. Crawler-type tractors seldom become stalled in the mud.
Fig. 16. — Portable pumping plant operating two ^-mile lines of 4-inch and
5-inch pipe with 61 sprinklers; capacity, about 1,100 gallons per minute.
(From Ext. Cir. 95.)
Pumps and Accessories. — Special lightweight centrifugal pumps have
been designed for use on portable systems. Although not heavy, they are
of rugged construction and are provided with an extra heavy shaft and
bearings that will withstand operation of the pump in any position.
Several sizes are available with capacities ranging from 200 to 1,200
gallons per minute, at heads of 80 to 200 feet. Detailed information con-
cerning the performance of these pumps can be secured from the manu-
facturers or from dealers handling sprinkling equipment.
Sprinkler pumps are usually mounted on the front or rear end of
the tractor, being supported by a special bracket welded or bolted to
the tractor frame. For accessibility, they should be as low as possible.
The pump may be driven by belts, chain, or gear. V-belts are better
adapted than flat belts because of the short distance between pulleys.
They should be properly selected to adequately handle the load at the
required speed. Manufacturers of V-belts publish complete tables giving
Bul. 670] Irrigation by Sprinkling 25
types and sizes to be used. Belts and chains should be protected against
the weather as well as water from the sprinklers. Where separate en-
gines are used, they are often directly connected to the pump with a
flexible connection.
The pump is equipped with a noncollapsible hose of adequate diameter
and length. These are available in two types, rough and smooth bore ; the
smooth type, which minimizes friction loss, is more desirable. Suction
hose should be of the same size as, or one size larger than, the connection
on the suction side of the pump, in accordance with the pump manufac-
turers' recommendations. The inlet end of a hose should be provided
with a screen, or strainer, to prevent debris from being drawn into the
pump. These strainers should have adequate openings which, even when
partially clogged, will not create an appreciable loss of head. Screens
of inadequate capacity may appreciably lower the pressure (increase the
vacuum) on the suction side of the pump and thereby decrease the effi-
ciency. To prevent sucking mud from the bottom of the ditch, the strainer
is usually designed with openings only on the top and rear end. These
strainers are generally fabricated by the manufacturer supplying
sprinkler pipe, or by the dealers handling this equipment. Many portable
plants have a derrick for raising and handling the heavy suction hose
and strainer when moving.
When sprinkler systems are to be used to distribute fertilizer or spray
solution, a 1-inch pipe connection between the suction hose and the pump
case should be provided to permit the introduction of the solution into
the system. Before using a sprinkler system for this purpose, one should
consider the corrosive effect of the solution on the pump and sprinkler
equipment. Ordinarily this is not serious if one takes the precaution of
thoroughly flushing the equipment before allowing to stand after use.
The pump discharge is equipped with a valve, which must be closed to
prime the pump. Where two lines are to be operated from the same
pumping plant, a Y- or T-fitting and two valves are required. This per-
mits one line to be operated at a time. Flexible discharge hoses connect
the pump and the sprinkler lines.
These hoses should be long enough (10 to 15 feet) to allow the ends
of the pipe to rest on the ground and to provide flexibility for making
the angles or turns in the lines. They should be provided with quick-
couplings of the type used on the sprinkler pipe.
Centrifugal pumps must be provided with some means of priming.
For this purpose, hand-operated suction pumps are often used. Some-
times an automatic priming device is connected to the intake manifold,
but it must be so arranged that the water is not drawn into the engine
when the pump is primed.
26 University op California — Experiment Station
Field Ditches. — Field ditches for supplying water to portable sprin-
kler systems must be fairly deep to submerge adequately the strainer
on the suction hose. In some places where sprinkling is extensive and the
water of suitable quality, ditches constructed primarily as drains are
used almost exclusively for this purpose. They are very convenient since
they usually require no portable dams or tappoons to hold back the flow
for the pump. The large ditches are usually constructed with a drag line
or a trenching machine.
Ditches constructed solely to serve a sprinkler system are usually
built with a ditcher having double-wing blades mounted on a subsoiler
frame. Large tractors, of at least 30 horsepower, are required to draw
these ditchers. Sometimes several trips through a ditch must be made to
bring it to adequate size, especially in level areas where velocities are
very low. Where the ditches are constructed on any appreciable grade,
portable tappoons of metal or canvas will hold back the flow and sub-
merge the strainer. Usually they are constructed with a weir notch or
opening of some type so that any excess flow will pass over without over-
topping the banks of the ditch. Occasionally seepage softens the banks
of the ditch, and the pumping plant becomes mired in the mud, especially
where pumps are mounted on trailers or trucks. In some places the
sprinkler system must always work down the ditch, with dams spaced
at frequent intervals so that the pumping plant is always adjacent to a
ditch that has had water in it for only a short time.
Where sprinkler systems are used on fields of irregular topography,
a strip of ground along the line of the ditch may have to be graded before
the ditch is constructed. This grading can be done cheaply in comparison
with the cost of leveling the whole field for surface irrigation. Since
ditches must be constructed in rather definite locations to serve a sprin-
kler system, they are not always practicable. Where the slopes are too
steep, or where adverse grades are encountered, the best solution is a
pressure supply line.
SYSTEMS USING STATIONARY PUMPS AND PRESSURE
SUPPLY LINES
Stationary Pumping Plants. — Stationary pumping plants are em-
ployed in connection with pressure supply lines. Where water is pumped
from a well with a deep-well turbine pump, the additional pressure re-
quired for sprinkling can be provided by additional stages (bowls) on
the pump. This arrangement, which eliminates the necessity of a booster
pump, is generally the most desirable. For an existing well with a pump
designed to lift the water only to the ground surface an auxiliary booster
pump may be more economical than changing over the turbine pump for
Bul. 670] Irrigation by Sprinkling 27
the higher heads required. A horizontal centrifugal pump of the same
type used on portable systems may serve the purpose. When connected
directly to the discharge pipe of the turbine pump, it requires no prim-
ing. The booster pump should have approximately the same capacity
as the turbine pump so that both will be reasonably efficient when op-
erating in series.
Stationary pumping plants are often electric-driven. Though the op-
erating cost may be somewhat higher than for internal-combustion en-
gines, the electric plant requires less attention — sometimes an important
consideration.
One disadvantage of electric-driven plants is the difficulty of chang-
ing the speed of the pump to accommodate the sprinkler system under
varying conditions, since electric-driven pumps are usually directly con-
nected and operate at a constant speed. With engine-driven pumps the
speed can be varied to produce the desired pressure at the sprinklers
when the length of supply line or the number of sprinklers varies. The
sprinkler system must be of such capacity and so arranged that adequate
pressures can be maintained under all operating conditions. Belt-con-
nected electric-driven pumps are sometimes advisable so that pulley
sizes can be changed to provide a change in speed, especially where large
fluctuations in the ground-water level are experienced during a season,
or where the number of sprinklers that can be used at one time varies
because of the irregular shape of the field.
Pressure Supply Lines. — As previously mentioned, pressure supply
lines may be either portable or stationary. The former usually consists
of extra lengths of portable sprinkler pipe without sprinklers, ordinarily
of larger size. They should be large enough to carry the flow of the sprin-
kler system without excessive friction loss. The approximate sizes re-
quired are as follows : for flows of 50 to 100 gallons per minute, 3-inch
pipe ; 100 to 200 gallons per minute, 4-inch pipe; 200 to 350 gallons per
minute, 5-inch pipe, and 350 to 600 gallons per minute, 6-inch pipe. Most
systems using 4-inch portable laterals will require 6-inch portable supply
lines. For systems with capacities of more than 600 gallons per minute,
portable supply lines are not satisfactory. Friction losses in pipe lines
are further discussed in a later section.
Several different kinds of pipe may be used for stationary supply
lines. Welded steel lines, which are most extensively used, are less ex-
pensive than any other kind of new pipe. Other pipes suitable for this
purpose include casing, standard pipe, cast iron, cement-asbestos, rein-
forced concrete, and reconditioned pipe and casing. Some of these mate-
rials, being rather expensive, are seldom used for agricultural sprinkler
systems.
28 University of California — Experiment Station
Stationary supply lines are sometimes laid on top of the ground, some-
times buried. When laid on the surface they must have expansion joints,
such as long-sleeve Dayton couplings, at certain intervals to take care
of the expansion and contraction caused by temperature changes. In
some corrosive soils steel pipe will last much longer if supported above
the ground on short piers than if laid directly on the ground or buried.
Such piers must be spaced close enough together so that the pipe will
not fail by bending, or crushing at the supports. Manufacturers can
usually supply specific information regarding safe plans for any par-
ticular kind of pipe. Surface pipe lines generally interfere with other
cultural operations, and are therefore objectionable.
Welded steel pipe is available in many sizes and weights. It is used
more extensively in sizes above 4-inch diameter. Ordinarily it is supplied
in 40-foot lengths and is connected together in the field by welding, spe-
cial couplings, slip joints, or flanges. Welded joints are the most com-
monly used. One main disadvantage of welding is that it destroys the
protective coating on the inside of the pipe at the joint. Sometimes an
outlet is welded on the pipe at the joint so that the inside of the pipe
can be recoated after welding. These outlets are then plugged. There are
two methods of field welding — eleetric-arc and acetylene. Electric-arc
welding being faster, is becoming more popular. Expansion joints are
usually spaced about 500 feet apart along the line, and at summits, or
near structures or branches along the line. Many kinds of fittings are
available for welded pipe.
Dayton or Dresser couplings are sometimes used for connecting
welded pipe. Although somewhat more expensive, they are especially
desirable for galvanized pipe (where welding would destroy the gal-
vanizing), and for small sizes, and short lines where it is not economical
to provide welding equipment. They also provide for the necessary ex-
pansion and contraction along the line. They are readily installed in the
field by common labor without special tools. Other types of special
couplings are available.
Slip joints are frequently used for connecting welded pipe, especially
where operating heads are fairly low. As they also eliminate the problem
of recoating the inside of the pipe after the joint has been made, they
are sometimes preferred to welding. Slip- joint pipe is provided with bell
and spigot ends. The bell end of the pipe is heated to expand it and to
soften the pipe coating, and the joint is made by either driving or jack-
ing the pipe sections together. When slip-joint pipe is installed above
the ground, it should be provided with hooks or lugs for wiring or bolting
the sections together to prevent them from pulling apart during tem-
perature changes.
Bul. 670] Irrigation by Sprinkling 29
Since welded steel pipe is fairly thin as compared with other types,
it should be protected against corrosion. The protection most commonly
used is an asphalt or tar coating, both outside and inside. Additional
protection usually consists of a wrapping; several grades are available.
The type most suitable for the particular soil conditions should be se-
lected. Welded pipe is sometimes galvanized. Although effective on the
inside, and on surfaces exposed to the atmosphere, galvanizing protects
for only a short time on the outside of pipe in contact with the soil. Under
some conditions "cathodic" protection may be desirable.6
Standard pipe with screw joints, used primarily for lines not exceed-
ing 4 inches in diameter, is available either plain (black) or galvanized.
Plain-end line pipe for welding or coupler connections is available in
many sizes and weights ranging from 2% to 24 inches outside diameter
(O.D.). Wall thicknesses range from approximately that of Merchant
casing to that of extra strong pipe.
Merchant casing, available in sizes ranging from 2}£ to 16 inches,
O.D., differs from standard pipe in being lighter in weight and in having
a finer thread. Casing may be connected either by threaded couplings or
by welding. Ordinary fittings, threaded for casing, are available. Con-
nections to valves and special fittings are made with flanges or by weld-
ing standard pipe nipples to the casing and using standard screw fit-
tings. Casing is usually installed with only an asphalt-dip coating or none.
Old pipe, casing, or boiler tubing, reconditioned, is used to some ex-
tent, especially for orchard systems in southern California. Usually it
costs less than any other pipe. Threaded joints are sometimes used, but
more often the joints are welded. Used pipe is reconditioned by cleaning
it to remove rust and scale ; defective sections are cut out, and an asphalt-
dip coating applied. Although the economy of reconditioned pipe may
be questioned, since there are no definite standards regarding its exact
condition when installed, it has often proved satisfactory. Where soils
are only slightly corrosive, it sometimes lasts fairly well. Its low cost has
been particularly attractive to farmers.
Plain concrete pipe as ordinarly used for irrigation systems lacks
strength to withstand the pressures required for sprinkling. Maximum
recommended working heads for concrete pipe range from about 20 feet
for large sizes to 50 feet for 6-inch pipe. It is suitable only for the so-
called "low pressure" sprinkler systems, and for lines supplying water
to pumping plants. Reinforced concrete pipe, however, can be made that
will withstand pressures adequate for sprinkling systems. Such pipe has
been used in some sections of California for irrigation systems, although
6 For a complete treatment of soil corrosion and pipe-line protection, see : Ewing,
Scott. Soil corrosion and pipe line protection. 277 p. American Gas Association, New
York, N. Y. 1938.
30 University of California — Experiment Station
very seldom in connection with sprinkling. In 12-inch and larger sizes
it is probably less expensive than steel lines and in addition is more
permanent.
Hydrants. — With stationary pressure supply lines, hydrants or valves
must be spaced at convenient intervals along the line. The usual arrange-
ment (fig. 13) shows the hydrants spaced three times the distance be-
tween portable lines so that each hydrant is used for three positions of
the portable line. In orchards, hydrants are usually spaced at every
second, or every fourth tree row. These hydrants commonly consist of a
short length of standard pipe and a valve. When welded steel lines are
used, the risers are usually welded to the line. Threaded tees in the line
are ordinarily used for risers on screw pipe.
Several different types of valves are suitable for hydrants. For small
sizes (3 inches or less) angle valves may be used; for hose connections
the garden- type angle valve with hose thread is suitable. For larger sizes,
gate valves are used almost exclusively. Valves are sometimes fitted with
quick-couplings for connection to the portable sprinkler lines, though
in many cases a portable fitting with quick-coupling is moved with the
portable line and screwed into the valve before the connection is made.
This arrangement eliminates a large number of special couplings and
somewhat reduces the installation cost.
To economize risers are sometimes provided with screw caps, and only
a limited number of valves are used. These valves are then changed about
once a day to the risers that will be needed for that day's operation.
Although this plan reduces the initial cost, it makes operation somewhat
less convenient, and necessitates shutting down the pumping plant while
the valves are changed.
LOW-PRESSURE SYSTEMS FOR FIELD CROPS
Most of the sprinklers used for portable systems except under-tree
orchard systems, operate best at pressures above 30 pounds per square
inch. With lower pressures, the distribution is usually less uniform.
Some sprinklers, however, distribute water satisfactorily at fairly low
pressures (10 to 20 pounds per square inch). As previously mentioned,
the area covered by a sprinkler increases with pressure up to a certain
point; and, therefore, higher pressures permit wider spacings of sprin-
kler lines with less frequent moves. High pressures also break up the
jet into finer drops, which have less tendency to puddle the soil, and
which permit higher rates of application. High pressures also make
possible the use of smaller pipe sizes — an important feature. When the
pressure is low, it is difficult to maintain uniform discharges from sprin-
klers along the line, especially on hillsides. Despite these disadvantages,
Bul. 670] Irrigation by Sprinkling 31
there are conditions under which low-pressure systems are desirable.
Since the power requirement and the cost of pumping are approximately
proportional to the pressure, the minimum pumping cost results with
the lowest pressure that is otherwise satisfactory. Investigation indi-
cates, however, that the power cost for pumping is usually only a small
part of the total cost of operating sprinkler systems. Probably more im-
portant than the cost of pumping is the fact that the lower pressure
permits larger-capacity systems to be used with a given power unit, and
this may appreciably lower the cost per unit of water applied, especially
if the larger system can be handled without an increase in labor cost.
Sometimes pressures of 10 to 20 pounds per square inch can be de-
veloped by gravity, whereas higher pressures would require pumping.
Sprinkling systems that can operate at these low pressures might be
feasible where those requiring higher pressures would not. Even though
the distribution of water with such systems might be less uniform than
with higher pressures, it might be better than with surface irrigation.
To meet these requirements, various types of low-pressure systems have
been developed. Some sprinklers operating at 15 pounds per square inch
can be spaced 40 to 50 feet apart. Low-pressure systems using sprinklers
of this type resemble in many respects the higher-pressure systems.
PERFORATED PIPE SYSTEMS
Another kind of low-pressure system consists of lightweight, per-
forated pipe that distributes water fairly uniformly over a strip of
ground along the line. This pipe is available in two types having rates
of application of about 1 inch and 2 inches per hour, respectively. An
interesting feature is that the actual rate of application for either type
remains approximately constant, while the effective width of the strip
covered varies with the pressure from about 20 feet at pressures as low
as 4 or 5 pounds per square inch to about 50 feet at a pressure of 20
pounds per square inch. In comparison with systems using rotating
sprinklers, perforated pipe systems have relatively high rates of appli-
cation which necessitates frequent moves of the pipe and may require
the operator to devote full time to this task. The alternate-line arrange-
ment, permitting continuous operation, is especially desirable. Because
of its light weight the pipe can be readily moved by one man. There is
also the advantage that a relatively large area can be irrigated with a
short length of pipe.
Perforated lines are used principally for irrigating pastures and other
low-growing crops. The high rates of application make them most suit-
able on pervious soils, or where only light applications are desired. Since
it is not possible to regulate and equalize the pressure along the line as
32
University of California — Experiment Station
with sprinklers, it is especially important that the system be arranged,
when possible, so that the line is along the contour rather than up and
down the hillside.
TRAVELING SPRINKLER MACHINES
To reduce the cost of sprinkling by reducing the amount of labor
required for operating the system, several growers in California have
constructed traveling sprinkler machines that pump water from a ditch
Fig. 17. — Traveling sprinkler machine in operation. This machine moves
continuously along the ditch at a speed of 75 feet per hour. Two sprinklers
discharge about 500 gallons per minute under pressure of 60 pounds per
square inch and effectively cover a strip 250 feet wide. The average applica-
tion is about 2y2 inches.
and distribute it through large sprinkler nozzles while moving slowly
but continuously along the ditch. These machines are planned for one-
man operation. Figure 17 shows an example. The capacity of these ma-
chines varies from about 400 to 800 gallons per minute. Most of them
have been built on a tractor geared down to move 1 to 5 feet per minute
with the engine operating at normal speed. To obtain maximum coverage
and to break up the jets from the large nozzles into a spray, pressures of
60 to 80 pounds per square inch are used. According to field tests on two
of these machines, they cover effectively a strip about 250 feet wide. They
require, therefore, the construction of ditches at this spacing through-
out the field, a feature which limits their use to rather flat areas where
such ditches are feasible. These ditches remove about 5 per cent of the
area from cultivation.
Bul. 670]
Irrigation by Sprinkling
33
Since portable pipe is not required, the cost of constructing such a
machine is generally less than the cost of an ordinary portable sprinkler
system of similar capacity. The cost of power for pumping is normally
higher than for other systems because of the higher pressure required.
For comparison, the total operating cost should include the cost of con-
structing the ditches plus the crop value on the land removed from
cultivation.
TABLE 1
Sprinkler Discharge Required for Various Applications of Water
in a Twelve-Hour Period
Sprinkler
Discharge, in gallons per minute, for various depths of application*
spacing,
feet
1.0
inch
1.5
inches
2.0
inches
2.5
inches
3.0
inches
3.5
inches
4.0
inches
4.5
inches
5.0
inches
6.0
inches
16 by 16
0.22
0.28
0.35
0.42
0.50
0.58
0.68
0.78
1.06
1.38
0 33
0.42
0.52
0.63
0 75
0.88
1 02
1.17
1.59
2.08
0 44
0.56
0.69
0,84
1 00
1.17
1.36
1.56
2.12
2.77
0.55
0.70
0.87
1 05
1.25
1.46
1.70
1.95
2.65
3.46
0.67
0.84
1.04
1.26
1.49
1.76
2.03
2 34
3.18
4.16
0.78
0.98
1.21
1.47
1.74
2 05
2.37
2.73
3.71
4.85
0.89
1.12
1.38
1.68
1 99
2.34
2.71
3.12
4 24
5.54
1 00
1.26
1.55
1.89
2.24
2.64
3.05
3.51
4.77
6.23
1.11
1.40
1 73
2 .10
2.49
2.93
3 39
3.90
5.30
6.92
1 33
18 by 18. . .
1 68
20 by 20
2 08
22 by 22
2 51
24 by 24
2 99
26 by 26
3 51
28 by 28
30 by 30
4.07
4.67
35 by 35
6.35
40 by 40
8.31
Average net application for the square area served by each sprinkler.
UNDER-TREE SPRINKLER SYSTEMS FOR ORCHARDS
Under-tree sprinkler systems have become very popular in some
areas. Most of these systems are portable or semiportable. Essentially
they consist of small, low-capacity sprinklers mounted on short risers
that are located in the area between adjacent trees. The sprinklers are
usually spaced the same distance apart as the trees so that each sprinkler
covers an area equal to that occupied by one tree. Three different types
are used for this purpose: whirling sprinklers; small, slow-revolving
sprinklers; and fixed sprinkler heads of the type used for concealed
lawn-sprinkler systems.
Sprinklers for Under-Tree Systems. — One problem in developing this
type of system has been to obtain a satisfactory sprinkler. To facilitate
the moving operations and to obtain adequate penetration without run-
off, these systems are sometimes operated on a 12-hour schedule, the
sprinklers being moved morning and evening (6 a.m. and 6 p.m.). For
the usual applications desired, this requires a sprinkler of rather low
capacity, discharging 1 to 3 gallons per minute. There has been some
34
University of California — Experiment Station
difficulty in obtaining sprinklers of these low capacities that perform
properly. Rotating and whirling sprinklers of low capacities are not
always dependable, for occasionally they fail to rotate. Also, because of
the very small nozzles required, clogging is a problem. With the fixed
sprinkler heads, it is difficult or impossible to secure the desired coverage
without increasing the capacity beyond that desired. Most of the fixed
Fig. 18. — Portable sprinkler unit illustrating small-sized
sprinkler pipe and typical sprinkler. The pipe is made from
iy2-inch, O.D., 18-gauge steel tubing, galvanized after fab-
rication. It comes in standard lengths of 20 feet with a
%-inch outlet on each length. Other lengths are also avail-
able.
sprinklers, such as lawn-sprinkler heads have, for example, capacities
between 2 and 3 gallons per minute at 20 pounds' pressure, and are de-
signed to cover effectively areas 12 to 16 feet square. Most tree spacings
are 20 feet or more, and sprinkler capacities of only 1 to 2 gallons per
minute are desirable. Fixed sprinklers that will effectively cover areas
20 feet square usually have capacities of 4 to 8 gallons per minute. They
are suitable, therefore, only on pervious soils and where sprinklers are
to be moved at frequent intervals. Table 1 gives the discharge required
for average applications of 1 to 6 inches in depth over square areas 16
Bul. 670]
Irrigation by Sprinkling
35
to 40 feet on a side in a period of 12 hours. This table covers the usual
range of tree spacings in orchards and assumes that sprinklers are to be
spaced the same distance as the trees — that is, the sprinkler is to be
located in each square between four adjacent trees. For the usual tree
spacing of 20 feet, an application of about 4 inches requires a discharge
of only 1.4 gallons per minute. This table can be used for other periods
of operation by simply increasing or decreasing the discharge accord-
ingly. If, for example, sprinklers are to be moved at 4-hour intervals,
Fig. 19. — Methods of determining and regulating the pressure at
sprinklers. At the left is shown a sprinkler riser equipped with shut-off
cock and "tank valve" which permits individual regulation of sprinklers.
At the right is a homemade Pitot gauge for determining the pressure at
the tip of a nozzle. The tip of the gauge is held in the center of the jet
about half the nozzle diameter from the end of the nozzle, and the pres-
sure, corresponding to the velocity head, is read on the pressure gauge.
three times the discharge given is required. From table 3 one can deter-
mine the approximate nozzle sizes for these discharges. Some orchard
sprinklers have two nozzles, and others have only one.
Portable Sprinkler Pipe for Under-Tree Systems. — Because of the
low capacity of under-tree sprinklers, sprinkler pipe in relatively small
sizes is suitable. Such pipe is now available in 1%- and 2-inch diameters.
It is made from 18-gauge tubing and has special quick-couplings (fig.
18) . Although ordinarily furnished in 20-foot lengths, other lengths can
be obtained on special order. Pipes of the same length as the tree spacing
are preferable. Special quick-couplings are also available for other kinds
of pipe, including lightweight electric conduit and hard-temper copper
tubing. Before the regular sprinkler pipe was available in small sizes,
many systems were made up from 1-inch and l^-inch electric conduit
(sometimes called electrical metallic tubing). This tubing is available
in 10-foot lengths in sizes ranging from % to 2 inches. Inside diameters
are slightly greater than the nominal sizes. It is available galvanized
36 University of California — Experiment Station
inside and outside or with an electroplated galvanizing on the outside
and lacquer finish on the inside. One objection to its use is that welding
or brazing the short lengths together destroys the protective finish on
the inside.
Sprinklers are mounted on ^-inch pipe risers 6 to 18 inches long, the
length depending upon whether or not a summer covercrop is grown in
the orchard. Risers should be as short as possible, since high risers tend
to overturn the pipe.
Where there is an appreciable variation in elevation along a sprinkler
line, or where there is excessive variation in pressure due to friction loss,
a shutoff cock on each riser, and some method of determining the pres-
sure, make it possible to regulate and equalize the pressure at the
sprinklers. One method is to use a small "tank valve," similar to an auto-
mobile-tire valve stem, between the shutoff cock and the sprinkler (left
view, fig. 19). Pressures can then be determined with a pressure gauge
equipped with an air-hose chuck. With some sprinklers, pressures can
be determined with a Pitot gauge (right view, fig. 19) . A whirling sprin-
kler is now available which automatically maintains a constant discharge
for all pressures above 8 pounds per square inch, and eliminates the
necessity for manual regulation.
The size of sprinkler pipe required for under-tree systems depends
upon the capacity and spacing of the sprinklers and upon the length of
the portable laterals. Where no pressure regulation is provided, the pipe
should be large enough so that the friction loss in the pipe will not exceed
20 per cent of the average pressure ; this will limit the variation in dis-
charge of the sprinklers to 10 per cent. The permissible length of pipe
in different sizes for different sprinkler discharges can be determined
from figure 26.
In small orchards one main supply line through the center with port-
able laterals extending halfway across the orchard is the simplest ar-
rangement. For large orchards, it is sometimes more economical to use
two parallel supply lines with shorter laterals, which will permit the use
of smaller portable pipe. One can determine the most economical ar-
rangement only after ascertaining what sizes of pipe are required under
both plans.
When sprinklers are moved only twice a day, two tree spaces can be
covered with each lateral per day, and the total number of laterals
required for the orchard is determined by the required frequency of
irrigation. Most orchards will require two or more portable laterals.
Stationary supply lines should be of ample capacity to carry the total
flow with a reasonable friction loss. Pipe of minimum size can be used
when the laterals are distributed throughout the orchard and arranged
Bul. 670]
Irrigation by Sprinkling
37
so that each takes care of the contiguous areas. Figure 20 shows a suitable
arrangement for a 10-acre orchard. Friction losses in different types of
pipe suitable for stationary lines are given in tables 5 and 6 and in
figure 24.
TJnder-Tree Systems Using Sprinklers Attached to Hose. — Many
under-tree sprinkler systems consist of several sprinklers mounted on
A/7/n. />/
27
assure at fiyc/ra/rt^y
te.per sp. //?.
2 'fe 'sfot/baary ma/'/T^
f'/z '/?yc/ra/?ts — (
Mm. pressure 20ft.x
\ /6 spr/Mers^O' Q 5 % %%
L apart . Capacity /•fyp./n.
\
1
VI
1
frees 20 ' a pari
9 & 6>6{jy0<££>iS>O &0£J[
f Pressc/re , 32 73. per s<?
]} Tote/ f/anfj £0 gpjn.
//7.
Fig. 20.-
-Arrangement of portable under-tree sprinkler
system for 10-acre orchard.
small stands joined with short sections of hose (fig. 21). One-inch or
%-inch garden hose is used for this purpose. Friction loss in hose of this
size is relatively high (table 9), as compared with pipe. This usually
limits the number of sprinklers that can be operated on each line to three
or four, according to the sprinkler capacity.
Hose systems are generally less desirable than those using metal pipe.
They require closer spacing of supply lines or the use of a long length
of supply hose so that several sprinkler settings can be made from the
same hydrant. When long hoses are used, a considerable friction loss
must be overcome by pumping against higher pressures. Hose systems
are less convenient to move than pipe systems, and the relatively short
38
University of California — Experiment Station
life of the rubber makes them fairly expensive. Until recently, sprinkler
pipe in small sizes was not available.
Portable Drag-Type Sprinkler Systems for Orchards. — Under-tree
sprinkler systems in which the units are moved by dragging lengthwise
have been used in some parts of California for several years. They were
developed to reduce the labor required for moving portable equipment.
They are of two general types — those in which the units are dragged
Fig. 21. — Under-tree sprinkler system consisting of three small lawn sprays
connected together with 20-foot lengths of %-inch garden hose. The entire
unit is moved from one supply lateral to another by dragging lenghtwise.
with a tractor or other source of power, and those in which they are
moved by hand. In the first type, the weight of pipe is not especially
important; standard pipe, boiler tubing, or casing can be used. One
system of this type used 2-inch O.D. boiler tubing welded into units 52
feet long (for 26-foot tree spacings) connected together with l^-inch
standard pipe couplings, short nipples of 1%-inch pipe being welded to
the ends of the tubing. This system has been operated with as many as
sixteen sprinklers spaced 26 feet apart on each lateral, although shorter
laterals with eight sprinklers proved more satisfactory. The head end
of the line is provided with a wheel cart to hold the risers vertical while
they are being moved. Pipe is moved with the same tractor used in culti-
vating the orchard. This system is operated on a 12-hour schedule, moves
being made morning and evening.
Bul. 670]
Irrigation by Sprinkling
39
For many orchards, however, it is more convenient to use smaller units
of such size and weight that they can be moved by hand. One system of
this type utilizes electrical conduit tubing connected together with a
special quick-coupling developed for the purpose. Units containing three
or four 20-foot lengths, with a sprinkler mounted on each, can be dragged
by hand. One principal advantage of systems of this type, is that one
need not walk on wet ground to move the pipe.
A portable drag-type unit consisting of %-inch type M copper tubing
has been developed for the University of California experimental or-
chard at Paradise, California. This unit is 120 feet long, permanently
connected together with solder fittings. It contains seven sprinklers 20
3/4' Aase swivel
connection - bose
S//6' brass screws nil 'A
was/iers and wing nt/ts
tm b
1-4
iny nipp/e-f/ttinj to O.PJ.
Heap Spvinkes
InT£PM£PiAT£ SpP/NKLESS
tfcAP SpPINKLEg
/■cr
/'xJ'nJ' wood strips
Sore Va'Ao/e asinj
'/t'fi/ier
Section A- A
Fig. 22. — Details of portable drag-type sprinkler unit made from copper tubing.
Either type of riser shown may be used, according to the type of sprinkler.
feet apart, mounted on short risers of the same material. Figure 22 shows
the details.
This hard-temper tubing, a relatively new product, is available in all
standard pipe sizes from ^-inch to 12-inch diameters. Its rigidity and
its light weight, together with its natural resistance against corrosion,
make it ideal for portable sprinkler units. The %-inch tubing has a wall
thickness of 0.032 inch and weighs only 0.328 pound per foot. All sizes
come with outside diameters % inch greater than the nominal size ; thus
%-inch tubing is % incn O.D. (outside diameter) and 0.811 inch I.D.
(inside diameter).
The Paradise units are connected to hydrants on a stationary supply
line with a 25-foot length of %-inch hose. The risers are held in a vertical
position by an adjustable wooden crossarm (at the head end), so they
can be kept vertical regardless of the cross slope. Brass or galvanized
bolts with wing nuts should be used in the crossarm, for iron bolts will
rust quickly under the sprinklers. The unit is designed to be operated
on a 12-hour basis and to be moved night and morning. The sprinklers
have a capacity of about 1.4 gallons per minute at 20 pounds' pressure,
40 University of California — Experiment Station
which provides an average application of about 4 inches in a 12-hour
period with sprinklers spaced 20 feet apart.
The operator moves the unit from one position to another by dragging
it forward, holding the sprinklers in a vertical position with the cross-
arm. With a main supply line every 140 feet, the hose can be left at-
tached to the unit. Where the supply lines are spaced twice the length
of the unit, the hose has to be alternated from one end of the unit to the
other. In this case, each end of the copper-tube unit should be equipped
with a swivel hose connection, and one end plugged, to eliminate the
necessity of exchanging the positions of the hose connection and the pipe
cap for each move. One extra plug for the hose connection will simplify
the moving.
When the unit has completed the last setting in a row, it is moved to
the next row in much the same manner as a freight train is switched from
one track to a parallel track. For alternate middle irrigation it is switched
over two tree rows. The tubing, being somewhat flexible, can be dragged
around a curve without injury. If bent too sharply, however, it will kink.
One-inch tubing is more satisfactory in this respect. Care must be exer-
cised, in moving from one row to another, to prevent the sprinklers from
catching on tree limbs.
Although the unit is easily moved from one tree row to another in a
deciduous orchard, difficulty might be encountered in a citrus grove
because of the low branches. This could be overcome, however, by using
standard pipe nipples for risers so that the risers and the sprinklers can
be removed before a change from one tree row to another. The use of tee
fittings with threaded side outlet eliminates the need for special cou-
plings at the top of the risers.
For winter storage the unit can be placed beside a fence, where it will
be out of the way and protected from damage. Since the entire unit is of
copper and brass, it will not corrode.
Copper tubing has been used for a portable sprinkler system by one
grower in southern California since 1938. It is ideally adapted to drag-
type units because it weighs less and has a higher permanent carrying
capacity than steel tubing. It requires no special protective coating,
such as the galvanizing that adds materially to the cost of sprinkler pipe
made from steel tubing. The drag-type system has the further advantage
of eliminating the need of quick-couplings. It is therefore less expensive
than most other portable sprinkler systems for orchards. Wear on the
bottom of the units is excessive in some places, especially on gravelly
soils.
A field test at Paradise indicated that the pull required to drag the
unit complete with the hose on wet ground was about 50 pounds.
Bul. 670] Irrigation by Sprinkling 41
SPRINKLING COMPARED WITH OTHER METHODS
OF IRRIGATION
Many claims made for sprinkling are not substantiated by facts or
have little practical significance. Sprinkling has, however, certain def-
inite advantages, some of which apply only under specific conditions.
Advantages of Sprinkling. — As was mentioned earlier, sprinkling
differs fundamentally from surface irrigation in that it distributes water
to the soil independently of the soil itself. With all methods of surface
irrigation the soil is the final medium for distributing the water, which
is either flooded over the surface or run in furrows spaced close enough
together so that most of the soil can be moistened. A large part of the
water applied may be lost by deep percolation, especially on pervious
soils with sandy or gravelly subsoils. Any soil can be sprinkled without
excessive waste. Unless the fields are properly graded and the irrigation
system carefully laid out, uniform distribution by surface methods is
difficult to obtain.
Sprinkling can sometimes be considered as crop insurance. During
spring months when drying north winds are common, crops are fre-
quently lost, or poor stands are caused by poor germination. Frequent
light applications at this time have beneficial results. Unless the crop is
planted in rows on raised beds, surface methods of irrigation are not
suitable for this purpose.
Frequently it is claimed that by sprinkling, one can obtain the same
penetration with less water. Extensive investigations have demonstrated,
however, that a certain amount of water is required for a given penetra-
tion regardless of the method. The moisture content of the soil must be
raised to the field capacity7 before further downward movement results.
Field capacity is the moisture percentage of the soil on a dry-weight
basis a few days after an irrigation or heavy rain, and is therefore
the approximate amount of moisture the soil will retain against the
downward force of gravity. Where a better penetration is apparently
secured with lighter applications by sprinkling, the water is simply more
uniformly distributed over the field and less is wasted. Excessive amounts
are not applied in low spots, and loss by runoff can be avoided.
Sprinkling eliminates many field ditches and permits the planting of
almost the entire acreage. Dispensing with ditches may in turn lessen the
weed problem. It also facilitates cultivation, making possible the working
of larger blocks.
7 For further discussion of field capacity and other terms relating to soil moisture
see: Veihmeyer, F. J., and A. H. Hendrickson. Essentials of irrigation and cultivation
of orchards. California Agr. Ext. Cir. 50:1-24. Eevised, 1936. (Out of print.)
42 University of California — Experiment Station
Under some conditions, sprinkling has proved beneficial in controlling
certain insect pests. In the Sacramento Valley it apparently aids in
controlling thrips* and red spider on beans, and probably helps control
both these pests on other crops. There is also some evidence that it reduces
the number of aphids on peas. Usually, however, it cannot be considered
a complete control; and where spraying or dusting is required, sprin-
kling may be detrimental because it tends to wash the residue from the
plants.
In the state of Washington" sprinkling was found to have no value
in the control of codling moths on apple — as believed by some growers;
instead, it interfered with the control by spraying. An average of 29 per
cent of previously applied arsenate of lead on apples was removed with
each sprinkling. The investigators also concluded that sprinkling did
not affect aphid infestation and leafhopper injury, though it did aid in
the control of red spider. Apparently, too, it was detrimental in connec-
tion with most apple diseases, such as perennial canker fruit rot, pear
blight, and downy mildew.
Portable sprinkler systems provide an economical method of applying
fertilizers in solution. The usual procedure is to dissolve a bag of fer-
tilizer in a barrel of water and introduce it into the sprinkler system
through a 1-inch hose connected to the suction side of the pump during
the first part of a run. Since the subsequent application of fresh water
completely washes the fertilizer solution from the foliage, no burning
results. Ammonium sulfate and calcium nitrate have both been success-
fully applied in this manner. Some of these materials, especially am-
monium sulfate, are corrosive to metals and one should be careful re-
garding their use, and especially to see that the system is thoroughly
flushed out after use. Frequent inspection of the pump and of the inside
of the pipe is suggested.
On land with rough or irregular surface features, sprinkling elimi-
nates the grading and leveling that would be required for surface irri-
gation. On shallow soils, especially those underlaid with hardpan, the
removal of a few inches of surface soil may be very detrimental. Under
such conditions, extensive leveling is not practicable, or, even where it is
feasible, a sprinkler system might cost less than such land preparation.
Tenant farmers would rather buy portable equipment than spend an
equivalent amount to level land they do not own.
In some parts of California sprinkling is used primarily as a protec-
tion against erosion. Where orchards have been planted up and down
hillsides, furrow irrigation down the slopes is often the only practicable
8 Bailey, Stanley F. The bean thrips. California Agr. Exp. Sta. Bui. 609 : 1-36. 1937.
n Overley, F. L., et al. Irrigation of orchards by sprinkling. Washington Agr. Exp.
Sta. Bui. 268:1-50. 1932.
Bul. 670] Irrigation by Sprinkling 43
method of surface irrigation. The result, frequently, has been excessive
erosion. Sprinkling combined with permanent covercrops may effec-
tively prevent further loss of soil. Although contour furrows can be
used for irrigating hillside orchards, that method necessitates planting
the tree rows on contour grades and is therefore not suited to orchards
planted in straight rows.
Conditions Favorable to Sprinkling. — Sprinkling may be an economi-
cal method of irrigation because of a low annual water requirement.
With portable systems the cost is primarily that of operation ; and where
the annual water requirement is low, sprinkling may cost less per acre
than suitable methods of surface irrigation involving heavier applica-
tions of water. In the Sacramento Valley for example, a spring crop
of peas may require only one light application, which might be made
more cheaply by sprinkling than by surface methods.
In the San Joaquin and Sacramento Delta, and along the Sacramento
River, sprinkling has found rather extensive use on areas where the
water table is close to the surface— that is, within 2 to 4 feet. In this
area the general slope is slight, but undulation of the ground makes
surface irrigation very difficult unless the land has been properly
graded. Because of the high water table, a continuous upward movement
of water by capillarity partially satisfies the crop requirements. "Without
irrigation, however, the upper 6 to 12 inches of soil usually gets dry, and
some crops suffer. Only light applications are required, however, to
moisten the surface soil ; and when surface irrigation is attempted, the
heavier applications required may temporarily waterlog the soil, with
detrimental results. Before the introduction of portable sprinkler sys-
tems, subirrigation was the method most commonly practiced. Water
was distributed to parallel ditches, or narrow trenches called "spud
ditches," spaced 50 to 200 feet apart, and was allowed to run until the
surface of the soil between the ditches appeared moist. As a result, the
water table was raised temporarily until all the soil above it was moist-
ened by capillarity. Subirrigation has several disadvantages. As a result
of evaporation and transpiration, salts are continuously moved upwards
and concentrated in the surface soil. When this effect is not offset by
adequate leaching during the winter, a detrimental saline condition may
develop. Also, because of the high water table maintained to make sub-
irrigation feasible, the rooting depth is restricted; better yields could
probably be obtained if the water table were kept lower. By sprinkling,
light but adequate amounts can be applied, and the surface soil kept
moist. Under these conditions the total annual water requirements are
low, usually less than half of what would be needed under similar
climatic conditions without the high water table ; and the cost of sprin-
44 University of California — Experiment Station
kling is not excessive even though the cost per acre-inch of water might
be high.
Sprinkling is also adapted to conditions where a shallow soil is under-
laid by either hardpan or gravel. With hardpan, because of the lighter
applications and more uniform distribution of the water, temporary
waterlogging can be avoided ; and with gravel, excessive losses of water
and leaching can be prevented.
Sprinkling is a desirable method of irrigation in certain coastal areas
where, because of moderate temperatures and high relative humidities,
the seasonal water requirements are low. In many such areas, rolling and
sloping ground makes surface irrigation somewhat difficult and expen-
sive.
Limitations of Sprinkling. — From the foregoing, it must not be in-
ferred that sprinkling is undesirable where surface methods of irriga-
tion are practicable. Sprinkling can be satisfactory under practically
all conditions although it is not always advisable. As compared with sur-
face irrigation, however, it offers few advantages in many places and
generally costs more. When fields are properly prepared for surface ir-
rigation, and where one irrigator can manage fairly large streams, the
cost of application by surface methods will usually be less than 25 cents
per acre-inch of water — sometimes as low as 5 or 10 cents. Sprinkling,
on the other hand, nearly always costs more than 50 cents per acre-inch
for labor and power and frequently more than $1.00, exclusive of inter-
est and depreciation. The total cost, including interest and depreciation,
and cost of water, varies greatly and is difficult to determine because of
the uncertainty as to the life of portable sprinkler equipment. Also,
because engines used for pumping generally serve other purposes as
well, only an arbitrary proration of cost can be made.
Besides the higher cost, other factors limit the use of sprinkler sys-
tems. Some soils absorb water so slowly that adequate applications by
sprinkling are not feasible ; only light applications can be made before
water accumulates on the surface. As a result, frequent irrigations are
necessary, with relatively greater evaporation losses and generally higher
costs than if heavier and less frequent applications were possible. With
light applications adequate penetration can be secured only by irrigating
while the soil still holds an appreciable amount of available moisture.
When water penetrates to only a shallow depth and the soil below this
depth dries out, the available soil nutrients are not utilized effectively.
During one season the writer observed the use of a portable sprinkler
system on a field of sugar beets on a deep loam soil that requires about
8 inches of water to wet the soil to a depth of 6 feet when dried to the
permanent wilting percentage. The winter rainfall was subnormal, and
Bul. 670] Irrigation by Sprinkling 45
only about 2 feet of soil was wet at the beginning of the growing season.
When irrigation started, the crop had extracted practically all available
moisture and was showing definite signs of wilting. The owner thought
best to hurry over the field before the crop was permanently damaged.
The sprinkler system, consisting of a line of 4-inch portable pipe with
sprinklers discharging about 20 gallons per minute mounted 40 feet
apart along the line, was operated in one position for about 2 hours, when
water would begin to accumulate on the surface. As this method of op-
eration resembled one which had proved successful in the nearby Delta
region, the owner was apparently following an established practice. Soil
samples taken at weekly intervals to determine the moisture conditions
indicated that penetration below about 18 inches was not obtained at any
time during the season. In many places the penetration did not exceed
12 inches. The system could not cover the field rapidly enough to prevent
the beets from wilting between irrigations. After each application, new
leaves would sprout out and grow vigorously; but soon these would wilt,
or even die and fall off. The yield was poor in comparison with that
previously obtained with surface irrigation. After one season's trial
sprinkling was abandoned. The writer believes, however, that satisfac-
tory results could be obtained on this soil with a sprinkler system of suf-
ficient capacity provided water was applied slowly enough to permit
adequate penetration. Investigations with sugar beets under similar
conditions show that two or three heavy surface applications, wetting the
soil to a depth of 6 feet, are sufficient and produce good yields.
On some soils pipe cannot be moved immediately after a heavy appli-
cation, because the ground becomes too soft on which to work. Light ap-
plications that permit immediate removal of the pipe are generally made.
Where feasible, a better solution is to use the alternate-line arrangement
and allow the pipe to remain on the wet ground for several hours before
moving it. When the system is of sufficient capacity to take care of the
field if operated only at night, the pipe can be left in place until the fol-
lowing evening and then moved for another run. This permits the soil to
dry enough to facilitate pipe moving. These methods require, of course,
relatively greater investments in equipment.
As previously mentioned, sprinkling sometimes spreads plant diseases,
and makes difficult the control of some insect pests. For some crops,
sprinkling cannot be used for these reasons.
COST OF SPRINKLING
When considering a sprinkler system, one must weigh all possible
benefits, together with the relative costs of sprinkling and other methods.
As mentioned above, advantages are sometimes, but not always, obtained
46 University of California — Experiment Station
from sprinkling. The farmer's individual problem must be considered.
The benefits in his specific case might include, for example, a saving
of water, insect control, higher yields, reduced cultivation costs. To
apply monetary values to these results is difficult or impossible without
specific information regarding the success of other sprinkler systems in
the vicinity.
The annual cost of sprinkling, which includes interest and deprecia-
tion on the investment, and operating costs, can be estimated for specific
conditions. The first step is to determine the initial cost, the investment
in equipment and installation. One can plan a system and then have
figures quoted by manufacturers, dealers, or contractors, or one can
obtain the services of an engineer experienced in these matters. Some
companies offer excellent engineering service and, given the necessary
information, will plan an economical layout and submit cost figures.
Depreciation and Interest on Investment. — The annual depreciation
should be figured separately for the different items making up the sys-
tem. The useful life and proper rate of depreciation applying to under-
ground stationary pipe will depend upon the corrosiveness of the soil
and upon the type of pipe used. Experience of water companies and
other utilities in the area may be drawn upon. Since portable sprinkler
pipe is relatively new, little experience data are available. Some of this
pipe in use for about ten years is still in good condition. A life of ten to
twenty years can be assumed for estimating depreciation. Sprinklers
generally last a much shorter time than pipe and sometimes require fre-
quent replacement of wearing parts. The life of different kinds of sprin-
klers vary; some have been completely worn out in less than one season
on portable systems. Specific information on the durability of the par-
ticular make being considered might be obtained from other users, or
from manufacturers or dealers. Generally speaking, an annual deprecia-
tion rate of about 25 per cent would be a fair estimate for sprinklers. The
proper rate of depreciation to apply to the pumping plant is question-
able. Where tractors are the source of power, they are generally used for
other purposes during part of the year. The depreciation chargeable to
sprinkling will depend upon the relative use made of the tractor for
other purposes. A good pump has a life of at least ten years. Electric
motors, when not overloaded, are probably good for at least twenty years.
In addition to depreciation one should include interest on the invest-
ment. Whether this should be figured at the rate one would have to pay
or at the rate he would receive on money invested will depend on the
individual case.
Cost of Operation. — Ordinarily the per-acre cost of portable sprinkler
systems is not high, and the total annual cost of sprinkling is not ap-
Bul. 670] Irrigation by Sprinkling 47
preciably affected by the interest and depreciation rates applied. The
principal cost is that of operation. In some instances, the annual op-
erating cost exceeds the total investment in the system. The operating
cost can be divided into two principal items: the power cost, including
the fuel, lubricating oil, and other materials required by the pumping
plant ; and the labor cost, for handling the system. In most cases the labor
cost is the principal item, sometimes four or five times higher than the
power cost.
Many systems with portable pumping plants are operated continu-
ously day and night, with two crews of two or three men, working 12-
hour shifts. These operators do nothing but take care of the sprinkler
system. The cost per acre-inch of water can be determined by dividing
the total daily operating cost by the number of acre-inches of water
applied. Assume, for example, that a system has a capacity of 500 gallons
per minute, that it is moved at 4-hour intervals, and that % hour is lost
at each move. The net daily operating time is therefore 21 hours. Remem-
bering that 450 gallons per minute is equivalent to 1 acre-inch per hour,
the total amount of water applied per day would be :
500X21 ooo
= 23.3 acre-inches.
450
Suppose that the system operated in the usual manner with two men
during the daytime and two men at night, at a daily wage of $4.00, or a
total of $16.00 per day for labor. Assuming the fuel for pumping costs
$4.00, and the lubricating oil and grease costs 50 cents, the total daily
operating cost would be $20.50, or 88 cents per acre-inch of water ap-
plied, of which 19 cents is for power and 69 cents for labor. If this system
was used 100 days during the season, the annual operating cost would be
$2,050, which would probably exceed the cost of the system, exclusive of
the tractor. These figures would be typical for many of the portable
sprinkler systems in the Sacramento Valley.
"Where a sprinkler system can be operated in such a way that the pipe
need be moved only twice a day, and especially where continuous atten-
tion is not necessary, the labor cost properly chargeable to the system
may be less than half of that given above.
In the summer of 1935 and 1936, field studies were made to obtain
some actual data on costs. The sprinkler systems studied were in the
Sacramento Valley and were all operated with portable pumping plants.
Tests were made to determine the capacity of the systems, and informa-
tion regarding the amount and cost of fuel and lubricating oil used and
wages paid was obtained from the operators or owners. From these data
the operating cost per acre-inch of water applied was calculated. Results
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are summarized in table 2, and following text explains details of deter-
mining data in the columns.
Explanation of Table 2. — Column 4 : The pressures at the sprinklers
were determined with a calibrated pressure gauge attached to a special
rubber fitting that was slipped over the main nozzle of the sprinkler. In
doing this, the discharge of one sprinkler nozzle was shut off, momen-
tarily increasing the pressure on the system. To obtain the correct pres-
sure, therefore, it was necessary to determine this increase in pressure
and subtract it from the gauge reading. The correction required, which
varied with different systems, was usually between 0.5 and 1.5 pounds
per square inch.
Column 5 : The discharge of the pump was determined by summing
up the discharge of the sprinklers, which was determined from the
sprinkler calibrations and from the pressure measurements at the
sprinklers.
Column 6 : Total head includes the pressure at the pump plus the lift
from the water surface in the ditch to the elevation of the sprinklers
plus the velocity head at the pump discharge, all expressed in feet of
head. The pressure at the pump was determined by adding the estimated
friction loss in the pipe between the pump and the nearest sprinkler, to
the pressure at that sprinkler.
Column 7 : The area covered per day is determined by multiplying
the area covered at each setting by the number of settings per day. The
area covered at each setting was equal to the width of the field (approxi-
mate length of the line) times the distance the line is moved. The number
of settings per day was either secured from the operators, or determined
from the frequency of the moves. The figure given, therefore, represents
the nominal area rather than the average area actually covered.
Column 8 : The average depth of application in inches is determined
by dividing the total application in acre-inches per day (column 9) by
the area irrigated per day (column 7).
Column 9 : The total application in acre-inches per day is determined
from the discharge of the system and the estimated net operating time
per day (column 11).
Column 10 : The efficiency of the sprinkler system is the ratio of the
total hydraulic energy at the sprinklers to the energy at the pump dis-
charge, expressed as a percentage. The difference between the figure
given and 100 per cent represents the loss of energy in the pipe by fric-
tion. Compare the average efficiency of the split-line arrangement and
the single-line arrangement.
Column 11 : The net operating time represents the approximate per-
centage of the total working day that the system was operating at full
Bul. 670] Irrigation by Sprinkling 51
capacity. For systems with a split line, where one line is operating while
the other is being moved, the loss of time in moving was taken as the time
required to move one line and the pumping plant.
Column 12 : Where the average daily wage included board and room,
an allowance was made for the value of the board and room in order to
make the total wages comparable for all systems studied. In nearly all
cases the daily wage was for 12 hours. These data are for 1935 and 1936,
a period of relatively low wages.
Column 13 : The average operating cost per acre-inch of water applied
for labor is equal to the total daily wages paid divided by the total ap-
plication in acre-inches per day (column 9) .
Column 14: The cost of power per acre-inch of water applied was
determined by dividing the total daily cost of fuel, lubricating oil, and
miscellaneous expenses (as given by owners or operators) by the total
output in acre-inches per day.
Column 15 : The total operating cost is the sum of columns 13 and 14,
no allowance being made for interest or depreciation on the equipment.
HYDRAULICS OF SPRINKLER SYSTEMS
Sprinkler systems are fairly complicated as compared with other ir-
rigation systems. They are composed of many different kinds of me-
chanical and hydraulic equipment. They require careful planning in
order to fit the fields on which they are to be used, and to be able to
supply the water required for the various crops. Pipe sizes, nozzle sizes,
pump sizes, and power requirements must be determined. Equipment
must be selected and installed. The sprinkler system as a whole must
be so proportioned that it will operate efficiently. The principal factors
that enter into the cost of operation must be clearly understood. The
planning of a sprinkler system is essentially an engineering function,
and one that requires, especially, a knowledge of the hydraulic principles
involved. The hydraulics of sprinkler systems is somewhat different than
the usual problems in hydraulics that are encountered in engineering
practice.10
The material covered in this and the following sections of the bulletin
have been prepared primarily for engineers, and others engaged in the
planning of sprinkler systems, and in the design, manufacture, and sale
of sprinkler equipment. The information is presented in a somewhat
10 Some phases of this subject have been discussed in previous papers; see: Chris-
tiansen, J. E. Irrigation by sprinkling. Agr. Engin. 18(12) : 533-38. Dec. 1937. Chris-
tiansen, J. E. Hydraulics of sprinkling systems for irrigation. Amer. Soc. Civ. Engin.
Trans. 107:221-50. 1942. Christiansen, J. E. The uniformity of application of water
by sprinkler systems. Agr. Engin. 22(3) : 89-92. March, 1941.
52 University of California — Experiment Station
technical manner; mathematical symbols and formulas, that would
otherwise be out of place, have been employed because they add to clear-
ness and conciseness.
DISCHARGE FROM NOZZLES
The discharge from all types of nozzles and orifices can be expressed
by the orifice formula, which is derived from Torricelli's theorem :
Q = CA V2gH (1)
where Q is the discharge in cubic feet per second; C is the coefficient of
discharge, which is approximately constant for a given nozzle or orifice;
A is the area of the nozzle or orifices in square feet; g is the acceleration
of gravity (32.2 feet per second per second) ; and H is the total head in
feet, which includes the velocity head. This formula is not convenient to
use, however, since the discharge from nozzles is commonly given in
gallons per minute, nozzle dimensions are expressed in inches, and heads
are expressed as pressures in pounds per square inch. Usually the veloc-
ity head in the riser is negligible in comparison with the pressure head
and no correction for it need be made. Expressed in these units, equation
1 becomes
q = 38.00 CayfP (2)
or
q = 29.85 Cd2\fp (3)
where q is the discharge in gallons per minute, a is the area of the nozzle
in square inches, d is the diameter of the nozzle in inches, and P is the
pressure in pounds per square inch. Table 3 gives the theoretical dis-
charge (C = 1.00) of round nozzles from %4 to Y2 inch in diameter. The
actual discharge of a nozzle can be obtained by multiplying the dis-
charge obtained from this table by the actual coefficient of discharge of
the nozzle.
Coefficients of Discharge of Nozzles and Sprinklers. — The coefficient
of discharge C of a nozzle is a ratio of the actual to the theoretical dis-
charge. It is the product of two other coefficients : the coefficient of con-
traction and the coefficient of velocity. The coefficient of contraction is
the ratio of the cross-sectional area of the jet, at the point of minimum
area, to the area of the nozzle. For most sprinkler nozzles the coefficient
of contraction is nearly 1.00. The coefficient of velocity for a sprinkler
is the ratio of actual velocity at the nozzle to the theoretical velocity that
would result if there were no friction losses in the sprinkler or nozzle.
This coefficient is an important factor, since it indicates the energy loss
due to friction and turbulence within the sprinkler. It is a measure of
co -* conco
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54 University of California — Experiment Station
the sprinkler's efficiency in converting pressure to velocity — a high value
indicating a small loss of energy. The coefficient of velocity for a sprin-
kler can be determined directly by dividing the pressure at the tip of the
nozzle, determined with a Pitot gauge (fig. 19) by the pressure at the
base of the sprinkler, corrected for velocity head and difference in ele-
vation. The same pressure gauge should be used for both pressure meas-
urements; if different gauges are used, small differences in the calibra-
tion of the gauges may cause an appreciable error in the coefficient.
The coefficients of discharge of well-designed sprinkler nozzles vary
from about 0.95 to 0.98. Precise determinations of the coefficient of
discharge are difficult ; and unless precision instruments are used in the
tests, determinations within 1 or 2 per cent are all that can be expected.
According to tests, coefficients of discharge for sprinklers with two
or more nozzles vary from less than 0.80 for some sprinklers to about
0.98 for others, the coefficient for any sprinkler depends upon the con-
struction of the sprinkler, the driving mechanism used, and the nozzle
sizes. Sprinklers with internal driving mechanisms generally have the
lowest coefficients ; in some cases there is an appreciable loss of energy
as the water passes through the sprinkler. The coefficient of discharge
for the same sprinkler may vary considerably with different nozzle sizes.
The larger the nozzles, the lower the coefficient, because of the higher
velocity and the greater energy loss within the sprinkler. With a %6-inch
main nozzle, for example, a certain sprinkler has a coefficient of dis-
charge of 0.97; with a %-inch main nozzle, a coefficient of only 0.88.
Discharge of Small Nozzles for Nozzle Lines. — The small nozzles used
on nozzle lines are constructed differently from those used on sprinklers.
They are more truly orifices in a thin wall. To minimize clogging, some
of them are machined with an abrupt entrance, the base of the nozzle
being slightly convex so that foreign matter will pass by without being
trapped. The outer end is countersunk so that the length of the drilled
hole is only about %2 inch. The abrupt entrance causes a contraction of
the jet, which passes through the nozzle without touching the sides of the
opening. This results in a crystal-clear stream that travels a maximum
distance. The length of the hole in some nozzles is sufficient so that the
jet expands and fills the opening at the discharge end. Although the
coefficient of discharge is higher, the result is a fuzzy jet that breaks up
more rapidly and does not carry so far.
Tests on several of these nozzles gave coefficients of discharge between
0.8 and 0.9, the low values being due to the abrupt entrances. The nozzles
tested had diameters varying from 0.034 to 0.045 inch, with discharges
varying from a minimum of 0.10 gallon per minute at 13.5 pounds per
square inch pressure, to a maximum of 0.34 gallon per minute at 42
Bul. 670]
Irrigation by Sprinkling
55
pounds per square inch. Table 4, giving the discharge of small nozzles
of this type at different pressures, is based on a coefficient of discharge
of 0.85, the approximate average value of the nozzles tested.
Pressure Required for Sprinkler Operation. — For proper perform-
ance, all sprinklers require a certain minimum pressure. Some are de-
signed for and operate best under fairly high pressures (more than 30
pounds per square inch), whereas others operate satisfactorily on pres-
sures of 10 to 15 pounds. Pressure is needed to provide the velocity
TABLE 4
Discharge of Small Nozzles for Use on Nozzle Lines*
Pressure,
pounds per
square inch
5
10
15
20
25
30
35
40
45
50
55
60
Discharge, in gallons per minute, for various nozzle diameters
0.030
inch
0.051
.072
088
102
114
125
135
144
153
161
169
177
0.035
inch
.120
.139
.155
.170
.183
.196
.208
.219
.230
0.240
0 040
inch
0.091
.128
.157
.181
.203
.222
.240
.256
.272
.286
.300
0.314
0.045
inch
0.115
.162
.199
.230
.257
.281
.304
.324
.344
.363
.380
0.397
0.050
inch
0.142
.200
.245
.283
.317
347
.374
.400
.425
.448
.470
0.490
0 055
inch
0.170
.240
.295
.343
.383
.420
.453
.485
.514
.542
.568
0 594
0.060
inch
0.204
.289
.353
.456
.500
.540
.577
.612
.645
.676
.706
0.070
inch
0.277
.392
.480
.554
.620
6S0
.734
.784
.832
.877
.920
0.960
0.080
inch
0.362
0.512
0.628
0.725
0.810
0.&88
0 959
1.025
1.087
1.146
1.201
1.255
0 090 0 100
inch inch
0.458
0.649
0.794
0.918
1.025
1.124
1.214
1.297
1.376
1.450
1.521
1.589
0.566
0.801
0.981
1.133
1.267
1.388
1.498
1.602
1.700
1.792
1.878
1.962
tested.
Based on a coefficient of discharge, C = 0.85, an approximate average value for different nozzles
which is required for two purposes : to secure distance of travel, or
coverage ; and to break up the water into small drops that can be prop-
erly distributed over the desired area. The first requirement can be satis-
fied only by a pressure adequate for this purpose. The second can be
satisfied by other means, such as deflectors or shape of nozzle, but only
at a sacrifice in the distance the water is thrown. Many sprinklers utilize
both methods, having one round nozzle for distance, and a second nozzle
with a deflector or slotted nozzle that breaks up the water into small
drops that fall near the sprinkler.
Let us consider what happens as the pressure is gradually increased
on a single round sprinkler nozzle. With very low pressure the water
issues in a solid stream, and all of it strikes the ground about the same
distance from the sprinkler. If the sprinkler rotates, only a narrow ring
receives water. With increase in pressure, the water becomes broken up
into drops and covers a larger and wider ring. Upon examining the
drops that strike the ground, we find that the largest are carried to the
56 University of California — Experiment Station
outside edge of the area covered, while the smallest fall near the sprin-
kler. With further increase in pressure the width of the ring becomes
equal to the radius, and the entire area receives water. As the pressure
continues to increase, more and more of the water falls near the sprinkler,
and the average size of the drops becomes smaller and smaller. The ef-
fective coverage of the sprinkler, which governs the permissible spacing
of sprinklers, will increase with pressure up to a certain point, and will
then decrease with further increase in pressure because of the larger
percentage of water falling near the sprinkler. The larger the nozzle,
the higher the pressure will be at maximum effective coverage.
The minimum pressure required for most sprinkler nozzles varies
from about 15 to 40 pounds per square inch. The larger the nozzles and
the greater the capacity of the sprinkler, the higher the pressure re-
quired for satisfactory distribution. Most field and orchard sprinklers,
except some under-tree sprinklers, operate under pressures between 20
and 60 pounds per square inch. Some very large sprinklers built for
special purposes require pressures of 100 to 150 pounds and cover
diameters up to 300 feet.
Special low-pressure sprinklers are available. Some of the small under-
tree orchard types operate on pressures as low as 5 pounds per square
inch and will cover adequately the area between adjacent trees.
The pressures mentioned above are those required at the sprinkler.
When pressures are determined at any point other than the sprinkler,
they must be corrected both for frictional losses and for differences in
elevation to obtain the actual pressure under which the sprinkler is
operating. Sometimes a user assumes that his system is operating under
the pressure indicated by a gauge at the pump or source of supply,
whereas some of the sprinklers may actually be working under a much
lower pressure.
FLOW OF WATER IN PIPE
The flow of water through a pipe is always accompanied by a loss of
head due to friction. Additional loss of head is also caused by sharp
bends, sudden enlargements or contractions in the pipe, or other ob-
structions. Figure 23 illustrates the effect of friction. The hydraulic
grade line is an imaginary line joining the elevation at which water
would stand in open columns connected with the pipe line. The height
of the hydraulic grade line above the pipe represents the head or pres-
sure at any point on the line. The friction loss is represented by a de-
crease in the elevation to which water will rise in the columns, or by the
slope of the hydraulic grade line. The friction loss does not always pro-
duce a decrease in pressure in the pipe line. If the slope of the pipe line
is greater than the slope of the hydraulic grade line, there will be an
Bul. 670]
Irrigation by Sprinkling
57
increase in pressure, that is (as illustrated in fig. 23), the total head or
pressure Hb above the pipe line at B, may be greater than the head Ha
at A. A measurement of the difference in pressure at two points on a pipe
line is, therefore, not a measure of the friction loss unless the line is
level, or unless corrections are made for differences in elevation.
Formulas for Friction Loss in Pipe Lines. — Friction losses in pipe
lines depend principally upon the roughness of the inside of the pipe,
the size of the pipe, and the velocity of the water. Many formulas have
been proposed to express the relation between these principal factors.
///j Head iosi
/h/ friction
Fig. 23. — Diagram illustrating friction loss in pipe lines.
See the text for explanation.
The formulas are based upon numerous tests on pipes of different kinds
and sizes. The three formulas most commonly used as a basis for tables
and diagrams for friction losses in pipe of the kind used for sprinkler
systems are Weisbach's, Williams and Hazen's, and Scobey's. "When
written in the same form, these formulas are as follows :
Weisbach's:11
Hf =
Williams and Hazen's :x
fLV*
2gD
Hi
3.022 LF1852
(J 1.862 _£) 1.167
(4)
(5)
11 This equation, sometimes called the Darcy formula, is mentioned in any standard
textbook on hydraulics.
12 This equation is commonly written V = 0.001"° 04 C R°-«* S0-*1, or V = 1.318 C
B° 63 S°-5i. See: Williams, Gardner S., and Allen Hazen. Hydraulic tables. 115 p. John
Wiley & Sons. New York, N. Y.
58
University of California — Experiment Station
TABLE 5
Friction Loss in Standard Wrought-Iron or Steel Pipe for C
100*
Flow, gallons
Friction loss, in feet of head, for each 100 feet of
nominal pipe sizes t
pipe in
the following
per minute
%
inch
1
inch
1M
inches
1H
inches
2
inches
2H
inches
3
inches
inches
4
inches
5
inches
6
inches
3
4.1
6.9
10.5
14.7
25.0
37.8
52.9
70.4
90.2
1.3
2.1
3.2
4.5
7.7
11.6
16.3
21.7
27.8
34.6
42.1
63.6
89.2
0.33
0.56
0.85
1.19
2.03
3.06
4.29
5.71
7.32
9.10
11.1
16.7
23.4
31.3
39.9
49.6
60.4
72 0
84.5
0.16
0.26
0.40
0.56
0.96
1.44
2.03
2.69
3.45
4.29
5.21
7.89
11.1
14.7
18.8
23.4
28.5
34 0
39.9
53 1
68.0
84.5
0.08
0 12
0.17
0.28
0.43
0.60
0.80
1.02
1.27
1.55
2.34
3.28
4.36
5.58
6.94
8.44
10.1
11.8
15.7
20 .2
25.1
30.5
42.7
56.8
72.9
90 5
0.12
0.18
0 25
0 34
0.43
0.53
0 65
0.98
1.38
1.83
2.35
2.92
3.55
4 24
4.97
6.62
8.48
10.6
12.8
18.0
23.9
30.6
38.0
46.3
70.0
98.0
;;;;;
0 09
0.12
0.15
0.19
0 23
0.34
0.48
0.64
0.82
1.01
1.23
1.47
1.73
2 30
2.94
3.66
4.44
6.23
8.30
11.3
13.2
16.1
24.3
34.0
45.3
58.0
72 0
87.6
0 07
0.09
0.11
0.17
0 24
0.31
0.40
0.50
0 61
0.72
0.85
1.13
1.45
1.80
2.19
3.07
4.09
5.24
6.51
7.92
12.0
16.7
22.3
28.6
35.5
43.4
51.6
60.5
80.5
0.09
0.13
0.17
0.22
0 27
0.33
0 39
0.46
0.61
0.78
0.97
1.18
1.66
2.21
2.83
3.54
4.28
6.47
9.06
12.1
15.4
19.2
23.3
27.9
32.7
43.5
55.8
69.3
84.2
0.07
0.09
0.11
0.13
0.15
0.20
0.26
0.32
0.39
0.55
0.73
0.94
1.17
1.42
2.15
3.01
4.01
5.13
6.38
7.76
9.27
10.9 '
14.5
18.5
23.0
28.0
4
5
6
8
10
12
14
18
18
20
25
30
35
40
45
50
55
60
70
0 08
80
0.11
90
0.13
100
0.16
120
0 23
140
0.30
160
0.38
180
0.48
200
0.58
250
0.88
300
1.23
350
1.64
400
2.10
450
2.61
500
3.17
550
3.79
600
4.45
700
800
900
5.92
7.58
9.31
1,000
11.44
* Computed from Williams and Hazen's formula.
t To convert head in feet to pressure in pounds per square inch, divide by 2.31.
Bul. 670]
Irrigation by Sprinkling
59
TABLE 6
Friction Loss in Standard Wrought-Iron or Steel Pipe for C t= 120*
Flow, gallons
per minute
45.
50.
55.
70.
80.
90.
100.
120.
140.
250.
300.
350.
400.
450.
500.
550.
600.
700.
800.
900.
1,000.
Friction loss, in feet of head, for each 100 feet of pipe in the following
nominal pipe sizes t
inch
2.9
4.9
7.5
10.5
17.8
27.0
37.7
50.2
64.3
80.0
97.3
ich
Si,
V4
inches
0.23
0.40
0.61
0.85
1.45
2.19
3.06
4.07
5.22
6.49
7.90
11.9
16.7
22.2
28.5
35.4
43.1
51.4
60.4
80.4
inches
0.11
0.19
0.29
0.40
0.68
1.03
1.45
1.92
2.46
3.06
3.72
5.64
7.90
10.5
13.5
16.7
20.3
24 3
28.5
37.9
48.5
60.4
73.4
2
inches
0.06
0.08
0.12
0.20
0.31
0.43
0.57
0.73
0.91
1.10
1.67
2.34
3.11
3.98
4.95
6.02
7.18
8.43
11.2
14.4
17.9
21.7
30.5
40.5
52.0
64.5
78.5
2V2
inches
0.08
0.13
0.18
0.24
0.31
0.38
0.46
0.70
0.98
1.31
1.67
2.08
2.53
3.02
3.55
4.72
6.04
7.52
9.14
12.8
17.0
21.8
27.1
33 0
50.0
70.0
93.0
3
inches
0.06
0.08
0.11
0.13
0.16
0.24
0.34
0.45
0.58
0.72
0.88
1.04
1.23
1.63
2.09
2.60
3.16
4.43
5.89
7.55
9.38
11.4
17.2
24 2
32.1
41.2
51.2
62.3
74.4
87.3
inches
0.05
0.07
0.08
0.12
0.17
0.22
0.29
0.36
0.43
0.52
0.61
0.81
1.03
1.29
1.56
2.19
2.92
3.74
4.64
5.65
8.54
12.0
15.9
20 4
25.3
30.8
36.8
43.2
57.4
73.6
91.4
4
inches
0.07
0.09
0.12
0.16
0.20
0.24
0.29
0.34
0.45
0.57
0.71
0.86
1.21
1.61
2.07
2.57
3.12
4.72
6.61
8.80
11.3
14.0
17.0
20.3
23.9
31.8
40.7
50.6
61.5
5
inches
0.05
0.06
0.08
0.09
0.11
0.15
0.19
0.23
0.28
0.39
0.52
0.67
0.83
1.01
1.53
2.15
2.86
3.66
4.56
5.54
6.11
7.75
10.3
13.2
16.4
20.0
inches
0.06
0.08
0.09
0.11
0.16
0.21
0.27
0.34
0.41
0.63
0.88
1.17
1.50
1.86
2.26
2.70
3.17
4.21
5.40
6.71
* Computed from Williams and Hazen's formula.
t To convert head in feet to pressure in pounds per square inch, divide by 2.31.
60 University of California — Experiment Station
Scobey's:13
KSLV l -9
Hf = TmD^ (6)
where Hf is the friction loss in length of pipe, L, in feet; V is the mean
velocity in feet per second; D is the diameter of pipe in feet; g is the
acceleration of gravity, about 32.2 feet per second per second ; / is the
Weisbach friction factor, which varies with smoothness, size of pipe, and
velocity, values ranging from about 0.015 for large smooth pipes to
about 0.050 for very rough pipes; C is Williams and Hazen's coefficient,
which varies with smoothness of pipe, values ranging from about 70 for
rough pipes to about 140 for very smooth pipe ; and Ks is Scobey's co-
efficient of retardation, which varies with smoothness of pipe, values
ranging from about 0.30 for smooth pipe to 1.0 or higher for very rough
pipe.
Friction Loss in Standard Pipe. — Table 5 gives the friction loss in
standard iron pipe as computed from Williams and Hazen's formula.
The losses are expressed in feet per 100 feet of pipe. This table is based
on a coefficient, C = 100, corresponding to iron pipe that has been in use
for ten to twenty years. This table is commonly employed for design
purposes. Table 6 is based on Williams and Hazen's formula with
C = 120. This table is believed safe for use with galvanized pipe under
ordinary conditions. Coefficients for new pipe normally range from 120
to 130; for old, badly corroded, tuberculated pipe they may be as low as
50 or 60. These very low values are due largely to a reduction in the
cross-section area of the pipe, since the coefficients are computed on the
full pipe area. The actual inside diameters and cross-section areas of
standard weight pipe are as follows :
Pipe size, Inside diameter, Area, square
inches inches inches
V2 0.622 0.30
% 0.824 0.53
1 1.049 0.86
1% 1.380 1.49
iy2 1.610 2.04
2 2.067 3.35
2V2 2.469 4.78
3 3.068 7.38
3V2 3.548 9.88
4 4.026 12.72
5 5.047 20.00
6 6.065 28.90
1:5 Scobey, Fred C. The flow of water in riveted steel and analogous pipes. U. S.
Dept. Agr. Tech. Bui. 150:1-136. 1930.
Bul. 670]
Irrigation by Sprinkling
61
The friction loss in pipe corresponding to other values of C can be
determined by multiplying the head loss, as given in table 5 for C = 100,
by the following factors :
C Factor
40 5.40
50 3.61
60 2.58
70 1.94
80 1.51
C Factor
90 1.22
110 0.838
120 0.713
130 0.615
140 0.536
joo 400 600 soo /poo /soo epoo
F/otv, go //oris per /nfm/te
5000 7,000 / 0,000
Fig. 24. — Friction loss in welded steel O.D. pipe. The double lines indicate differ-
ences in friction loss between pipes of the lightest and heaviest wall as commonly
made.
Friction Loss in Welded Steel Pipe. — Figure 24 shows the friction
loss in welded steel pipe of the type commonly used for main supply
lines for portable sprinkler systems. This graph is based on Scobey's
formula, with the coefficient K s = 0.32, which corresponds to new pipe
in good condition. Since welded steel pipe is relatively thin, it is usually
protected against corrosion by galvanizing or with an asphalt or tar
coating, with or without a protective wrapping. The effectiveness of
these coatings in maintaining a high carrying capacity depends entirely
upon the material used and the corrosiveness of the water. In designing
stationary pipe layouts for sprinkler systems, one may well add a
62
University of California — Experiment Station
certain amount to the required flow, to serve as a factor of safety ; an
allowance of 10 to 20 per cent is usual.
Friction Loss in Copper Tubing. — Copper tubing (also called copper
pipe) is available in both soft and hard temper. It is made in four stand-
TABLE 7
Friction Loss in Type M Copper Tubing*
Flow, gallons
Friction loss, in
feet of head, for each 100 feet of tubing
following nominal sizesf
n the
per minute
inch
inch
1
inch
inches
. 1H
inches
2
inches
1
1.7
6.1
14.0
22.2
34.0
47.0
80.0
1.1
2.4
4.0
6.1
8.5
14.4
21.9
30.5
40.8
52.1
64.8
78.8
0.7
1.2
1.8
2.4
4 1
6.3
8.8
11.8
15.0
18.7
22.7
34.3
48.2
64.0
0.42
0 63
0.88
1.51
2.28
3.18
4.23
5.43
6.75
8.22
12.4
17.4
23.1
29.6
36.8
44.8
62.9
0.28
0.39
0.66
1.01
1.42
1.87
2.40
2.99
3.63
5.51
7.71
10.2
13.2
16.3
19.8
27.8
37.0
47.3
59.0
71.6
2
3
4
5
6
0.10
8
0.17
10
0.27
12
0.37
14
0.49
16
0.63
18
0.78
20
0.95
25
1.44
30
2.01
35
2.67
40
3.42
45
4.26
50
5.18
60
7.25
70
9.64
80
12.32
90
15.36
100
18.68
* From Williams and Hazen's formula, C = 140.
t Actual inside diameters are given in table 8.
ardized types O, M, L, and K, classified according to wall thickness.
The outside diameters of all types are % inch greater than the nominal
sizes. One-inch tubing is, for example, 1% inches O.D., and varies from
1.055 inches I.D. for type M to 0.995 inch for type K. The lightest weight,
type O, is available only in sizes above 3 inches.
Since the inside diameter of copper tubing is usually less than the
inside diameter of iron pipe of the same nominal size, the friction loss
is sometimes greater than for iron pipe. This is particularly true of the
Bul. 670]
Irrigation by Sprinkling
63
heavier-walled tubing, types L and K, and of Vi-inch and l^-inch sizes.
For this reason friction loss tables for iron pipe are not suitable for
estimating the carrying capacity of copper tubing.
Tests to determine friction losses in pipe of various kinds generally
indicate a lower value for the exponent of V for smooth pipes, than for
rough pipes. Dawson's1' formula for flow of water in copper pipe can
be written :
TT 0.000315 LV1-75
H' = ^TS ' (7)
This equation corresponds approximately to Williams and Hazen's
formula with C = 140. Tests by A. F. Pillsbury15 on %-inch copper
tubing indicate slightly lower losses than given by Dawson's formula,
but they verify this exponent of V. Table 7, computed from Williams
and Hazen's formula, with C = 140, gives the friction loss in type M
copper tubing, sizes 0.5 to 2 inches. Table 8 compares the inside diameters
TABLE 8
Comparison of Inside Diameters and Friction Losses in Copper Tubing,
Types M, L, and K*
Type M (light)
Type L (medium)
Type K (heavy)
Nominal size,
inches
Inside
diameter,
inches
Friction
factor
Inside
diameter,
inches
Friction
factor
Inside
diameter,
inches
Friction
factor
y2
0.569
0.811
1.055
1.291
1.527
2.009
1.00
1.00
1.00
1.00
1.00
1.00
0.545
0.785
1.025
1.265
1.505
1.985
1.23
1.17
1.15
1.10
1.08
1.06
0.527
0.745
0.995
1.245
1.481
1.951
1.52
5i
1.52
1
1.33
\\i
1.19
\y%
1.16
2
1 15
* The friction loss in copper tubing, types L and K, can be found by multiplying the loss given in
table 7 by the friction factor given in this table.
and friction loss in types M, L, and K. To determine the friction loss in
types L or K it is necessary to multiply the friction loss given in table 7
by the proper friction factor from table 8.
Friction Loss in Garden Hose and Hydrants. — Garden hose in %-
and 1-inch sizes is used to a large extent in connection with sprinkler
systems for orchards and also for lawns. In general, friction loss through
hose of this type is appreciably higher than for iron pipe and probably
varies with different makes of hose. According to recent tests at Davis,
14 Dawson, F. M., and J. S. Bowman. Interior water supply piping for residential
buildings. Wisconsin Engin. Exp. Sta. Bul. 77:1-54. 1933.
15 Unpublished data.
64
University of California — Experiment Station
the friction loss varies with the pressure, the loss becoming less as the
hose swells under pressure. Table 9 gives the friction loss in 100 feet of
hose and the loss through garden valves of the type generally used as
hydrants for small sprinkler systems.
FLOW OF WATER IN PIPES WITH MULTIPLE OUTLETS
The friction loss in pipe lines with sprinklers or nozzles spaced equi-
distant along the line can be calculated in a step-by-step process, comput-
ing the loss for each length between adjacent sprinklers for which the
TABLE 9
Approximate Friction Loss in Garden Hose and Garden Hydrants*
Friction loss, in pounds per square inch, for:f
Flow,
gallons per minute
100 feet
of %-inch
hose
100 feet
of 1-inch
hose
%-inch
garden
hydrant
1-inch
garden
hydrant
5
6.8
9.6
12.8
16.0
20.0
24 4
34.0
45.2
58.0
72.0
88.0
1.6
2 0
2.8
3.6
4.4
5.6
8.0
10.4
13.2
16.4
20.0
24.0
28.0
32.4
37.2
42.0
0.3
0.4
0.6
0.8
1.0
1.2
1.7
2.4
3.1
3.9
4.8
5.8
6.9
8.0
9.2
10.6
0 1
6
0.2
7
0.2
8
0.3
9
0 4
10
0 5
12
0.7
14
0.9
16
1.2
18
1.5
20
1.9
22
2.3
24
2.7
26
3.2
28
3.7
30
4.3
* Friction losses in hose and valves vary widely with different makes of equipment.
These values, based on tests at Davis, should be used only as a guide in determining the
size required.
t To convert pressure in pounds per square inch to head in feet, multiply by 2.31.
flow is constant. Such calculations are tedious, especially for long lines.
Starting at the distal end of the line with an assumed pressure and sprin-
kler discharge, one must work back toward the source to determine the
required pressure and the total flow for the line. By making certain
approximations, however, one can obtain a simple and fairly accurate
solution.
Derivation of Equations for Calculating Friction Losses. — If the line
is level, the pressure will be a minimum at the distal end of the line,
and will increase gradually toward the source. The discharge of the end
sprinkler will be a minimum, and, as we proceed toward the pump, each
sprinkler in turn will discharge slightly more water because of the in-
crease in pressure. The variation in the discharge of the sprinklers is
Bul. 670] Irrigation by Sprinkling 65
ordinarily not great, and to simplify the calculation of the friction loss,
we will assume that the discharge of each sprinkler on the line is equal
to the average discharge of all of the sprinklers, that is
qi = Q2 = q* = qa = — (8)
when qlf q2, and qs are the discharges of the first, second, and third
sprinklers respectively, and qa is the average discharge of all the sprin-
klers which is equal to the total discharge Q divided by the number of
sprinklers, N. A relation between the friction loss in lines with multiple
outlets and ordinary pipe lines, where all of the water is carried to the
end of the line, can then be derived from a general equation for flow of
water in pipe lines. Writing equations 4, 5, and 6 in a generalized form,
but
and
'. - ^
A — D2
4
F- = K^'
D2m
Making this substitution for Vm, and combining Kx and K2m, we get
For pipes with multiple outlets, the total friction loss is equal to the
sum of the losses between adjacent outlets. Letting qa equal the discharge
at each outlet, and 8 equal the spacing between outlets, the friction loss
between the last two outlets at the distal end of the line becomes
j, KSqam nu
and the loss between the next two outlets is
_ KS(2qa)<* _ KSgSV
2 £)2m+n £)2m+n ^ '
and, similarly
KSgamNm
k" = D2m+n • (13)
66
University of California — Experiment Station
The total friction loss for any number of spaces N, between adjacent
outlets, becomes
#/ = 2(/i1 + /i2+ • • •+hN)=KSqam2(lm + 2m + Zm+ • • ■+#*). (14)
L Q
Substituting — for S, ^ for qa, and 3>Nm for 2(lw+ 2W+ 3W+ • • • + Nm)
we have
K (L\/Qm\ _ 2Nm /KLQm\ _ (KLQm\
Comparing this equation with equation 10 we see that the friction loss in
a pipe with multiple outlets can be determined by first estimating the
TABLE 10
Values of the Factor F* by Which the Friction Loss in Pipe
Must Be Multiplied to Obtain the Actual Loss in a
Line With Multiple Outlets
Number of outlets
• m = 1.85
to = 1.90
to = 2.00
1
1.0
0.639
0.535
0.486
0.457
0.435
0.425
0.415
0.409
0.402
0.397
0.394
0.391
0.387
0.384
0.382
0.380
0.379
0.377
0.376
0.374
0.372
0.370
0.369
0.368
0.365
0.364
0.361
0.356
0.351
1.0
0.634
0.528
0.480
0.451
0.433
0.419
0.410
0.402
0.396
0.392
0.388
0.384
0.381
0.379
0.377
0.375
0.373
0.372
0.370
0.368
0.366
0.364
0.363
0.362
0.359
0.357
0.355
0.350
0.345
1.0
0.625
0.518
0.469
0.440
0.421
0.408
0.398
0.391
0.385
0.380
0.376
0.373
0.370
0.367
0.365
0.363
0.361
0.360
0.359
0 357
0.355
0.353
0.351
0.350
0.347
0.345
0.343
0.338
0.333
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
22
24
26
28
30
35
40
50
100
00
2A7m
F = Nm+l , as given in equation 14.
Bul. 670] Irrigation by Sprinkling 67
friction loss in the line, assuming that all the water is carried to the end
of the line, and then multiplying this loss by a factor, F, that depends
on the number of outlets on the line and the value of m used in the fric-
tion loss formula. Any suitable formula, table or graph may be used,
therefore, for estimating the friction loss in lines with multiple outlets.
Values of the factor F, have been calculated, and are given in table 10
for m = 1.85, 1.9, and 2.0. Values of this factor for any other value of
m can be readily computed from the approximate expression18
F = -J— + -L+^™E±. (15)
m + 1 2N QN2 K J
According to tests on sprinkler pipe with sprinklers uniformly spaced
along the line the friction factor K is lower, and the exponent m is
slightly higher, than for ordinary pipe flow, because of the partial
recovery of velocity head at each outlet. For many problems, a value
of m = 2 can be used.
Friction Loss in Sprinkler Lines. — Most portable sprinkler pipe is
made from lightweight O.D. tubing. A special graph, figure 25, shows
the friction loss in 16- and 18-gauge sprinkler pipe in sizes 1 to 6 inches
O.D. This graph is based on Scobey's formula, with coefficient Ks as
given. According to tests on such pipe, higher values of Ks may be ex-
pected of the smaller sizes, and the values are influenced by the type of
coupling. The values given are believed conservative; most sprinkler
pipe will have less friction loss than is shown by the graph. Some of the
tests indicated that the pressure drop for each section of pipe between
adjacent sprinklers is approximately proportional to the square of the
velocity (V2) in that section of pipe, as given in Weisbach's formula,
instead of some lower power of V as given in Williams and Hazen's or
Scobey's formulas. The reason for this is, that the velocity head is par-
tially recovered at each sprinkler outlet where the mean velocity in the
pipe is suddenly reduced. For estimating the friction loss in a line of
sprinkler pipe with sprinklers, it makes little difference whether Weis-
bach's or Scobey's formula is used ; but for estimating the loss in a line
without sprinklers, Scobey's formula is preferable, unless different
values of / are selected for different velocities. Where calculations
from a formula are required, Weisbach's formula will be found easier
to use. For sprinkler pipe, the following values of / are suggested :17
18 This expression is exact for m = l, and m = 2, and very nearly correct for
m = 3 and intermediate values. Where N is greater than 10, the last term in the
expression is negligible and can be omitted.
17 Based upon field and laboratory tests on several makes of sprinkler pipe, and
supplemented by published data, principally from Pigott, E. J. S. The flow of fluids
in closed conduits. Mech. Engin. 55(8) : 497-501, 515. August, 1933.
68
University of California — Experiment Station
1-inch, 0.028; 1%-inch, 0.025; 2-inch, 0.023; 2%-inch, 0.022; 3-inch,
0.021; 4-inch, 0.020; 5-inch, 0.019 ; and 6-inch, 0.018.
To simplify further the problem of determining friction losses in
sprinkler pipe, a logarithmic chart, figure 26, has been prepared. This
combines data given in figure 25 and table 10. Although based on
a sprinkler spacing of 20 feet, it can be used for any spacing if the
^ 6
X
•5 0.4
—f-
f
zl :
r
t-
—7
^===;
/
r
/ /
J
z_ Z
f
I i
^ /
T
-7 ^y
/
1
j
/
f~
A 4-
\t
/
T
7_
/
/ i
-/
4/
M
7
f
/
/
L ,
7
/ ,
V
f %
/ /
/
/
i
\
7
f
\
i
7
A
~r
~4
/
;c
Ja
r
/
/
Y V
/--
f
t
'
~l X
/
— >
/
-f
4 1
—4-
7=-
7-
/ -
r
1
/
' (
-1-
/
t —
f Sco6ey's Forma /a
1 with iro/i/es of
/
/
f
-7-
/
/ /fs os g/ven. |
/
/
/
~l_
1 i
f
1 II II
/5 X JO 40 SO 60
r~/o»v, ga//o/?s per /nirwle
600 0OO /pOO
Fig. 25. — Friction loss in portable sprinkler pipe. For mains and supply lines,
where all of the water passes through all of the pipe, this diagram is sufficient. For
lines equipped with sprinklers, it must be used in conjunction with table 10.
friction loss obtained from the diagram be multiplied by the ratio:
& To determine, for instance, the friction loss in a
20
600-foot line of 3-inch pipe with sprinklers discharging 11 gallons per
minute, spaced 40 feet apart :
600
1. There will be -^r = 15 sprinklers.
2. Enter left side of diagram for q = 11 ; follow to right to intersection
with 3-inch pipe.
3. Drop vertically down to a point half way between the lines for
14 and 16 sprinklers.
4. Follow to the right margin, and read friction loss of 4.1 pounds per
square inch.
S>?/?£//U/ JP(/ Pi/Of/pS '6 '$jfijPl/3S'/p J2/)/l//J(/s> pfcPJ^/lj/
^
CD
a
rt
a
t->
o
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^
-M
■+-»
fcJD
3
o
ft
03
OQ
V
CD
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r*l
o
2
C 1
H3
ft
CO
w
«H
a
o
ft
be
t»
g
o
03
3
ft
a
•rH
f-l
ft
rt
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A
M
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r*i
fl
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03
ft
fat)
OQ
03
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^
s
o
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eS
-t-3
on
DQ
a
O
o
Ph
fl
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rr!
CD
ej
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co
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he
o
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^
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ft
Rl
-u
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a
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fl
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fl
-^
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02
o
hrl
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rW
'.
PI
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eg
ft
bo
h
PR
^J
70 University of California — Experiment Station
40
5. Multiply this by ~~ = 2 to obtain correct loss for 40-foot spacing :
4.1 X 2 = 8.2 pounds per square inch.
When the size of pipe is reduced along the line, that is, when two or
more sizes of pipe are used, the problem of determining the friction loss
is more difficult. The procedure can best be explained by an example.
Assume that a 1,000-foot line is composed of 320 feet of 3-inch pipe, 360
feet of 4-inch pipe and 320 feet of 5-inch pipe, with a total discharge of
375 gallons per minute. With sprinklers spaced 40 feet apart, a total
of 25 sprinklers, the average discharge of the sprinklers would be 15
gallons per minute.
320
From figure 26, the friction loss in 3-inch pipe with -j- = 8 sprin-
klers would be about 1.4 pounds per square inch. This is multiplied by 2
to obtain the actual loss because the sprinkler spacing is 40 feet instead
of 20 feet. The corresponding loss for 8 sprinklers, spaced 20 feet apart
on 4-inch pipe is less than 1 pound per square inch, and cannot be de-
termined from the graph. By extending the lines beyond the lower edge
of the graph, this loss is estimated to be 0.5 pounds per square inch which
makes the actual loss about 1 pound. The excess loss in the 3-inch pipe,
compared with the 4-inch pipe, will be 2.8 — 1.0 = 1.8 pounds. The loss
in 680 feet of 4-inch pipe, with 17 sprinklers, is 2 X 2.4 = 4.8 pounds.
The actual loss in the combination 3- and 4-inch line is then
1.8 + 4.8 = 6.6 pounds. The corresponding friction loss in 680 feet of
5-inch pipe would be approximately 2X1 = 2 pounds. The excess for
the 3- and 4-inch line as compared with 680 feet of 5-inch line is, there-
fore, 6.6 — 2=4.6 pounds. The loss in 1,000 feet of 5-inch pipe (25
sprinklers) would be 2 X 2.2 = 4.4 pounds. The total loss in the 1,000-
foot combination line is, therefore, 4.4 + 4.6 = 9.0 pounds which is
about the maximum permissible for efficient operation. Had 4-inch pipe
been used for the entire line, the loss would have been 2 X 7.2 = 14.4
pounds per square inch.
Friction Loss in Nozzle Lines. — For nozzle lines of standard galvan-
ized pipe, one can estimate the friction loss in a similar manner. Assum-
ing that all the water flows to the end of the line, the friction loss can
be determined from tables 5 or 6. This loss is then multiplied by the
proper factor F (table 10) for m = 1.85 to obtain the actual friction
loss in the nozzle line. To determine, for example, the friction loss in 100
feet of 1-inch pipe with nozzles having an average discharge of 0.25
gallon per minute, spaced 3 feet apart, the steps would be as follows :
0.25 X 100
1. Determine the total discharge, which is equal to — — = 8.3
gallons per minute.
Bul. 670] Irrigation by Sprinkling 71
2. Using table 6, for C = 120, we find that the friction loss Hf for 8.3
gallons per minute in 100 feet of 1-inch pipe is about 6.0 feet of head.
3. From table 10, m = 1.85, interpolating for 33 nozzles, F = 0.366.
4. The actual friction loss is, therefore, approximately 0.366 X 6 = 2.2
feet.
When several pipe sizes are used in a nozzle line, the problem of de-
termining the friction loss is essentially the same as for a combination
portable sprinkler line. Assume that a nozzle line is composed of 100 feet
of 1%-inch pipe, 100 feet of l^-inch pipe, and 100 feet of 1-inch pipe,
with nozzles delivering 0.25 gallon per minute spaced 3 feet apart. The
total discharge would be 25 gallons per minute; 8.3 gallons per minute
at 100 feet ; and 16.7 gallons per minute at 200 feet. The friction loss in
the 100 feet of 1-inch pipe, computed above, is 2.2 feet of head. Had this
been l^-inch pipe, the loss would have been 0.366 X 1.6 = 0.6 feet. The
excess loss for 1-inch pipe, as compared with 1^-inch pipe, is therefore
2.2 — 0.6 = 1.6 feet. For 200 feet of 1%-inch pipe, the loss would be
0.36 X 5.6 X 2 = 4.0 feet; for 200 feet of 1%-ineh pipe, it would be
0.36 X 2.6 X 2 = 1.9 feet. The excess for l^-inch pipe compared with
1%-inch pipe is 4.0 — 1.9 = 2.1 feet. For 300 feet of 1 Ms-inch pipe, the
loss would be 0.35 X 5.6 X 3 = 5.9 feet. The actual loss for the combina-
tion line would be the loss for 300 feet of l^-nich pipe plus the excess
losses due to reducing the size, or 5.9 + 2.1 + 1.6 = 9.6 feet of head.
This method of computing losses, though only approximate, is fairly
reliable when the average nozzle discharge is known or can be deter-
mined. In order to limit the variation in discharge of the nozzles to 10
per cent, nozzle lines should be so designed that the friction loss does not
exceed 20 per cent of the average operating head on the line.
Discharge, Pressure, and Power Requirement Relations for Sprinkler
Lines. — Certain fundamental relations between pressure, discharge, and
power requirement for sprinkler lines apply to all types of sprinkler
systems. Although the friction loss in sprinkler lines depends only upon
the flow of water in the pipe, this depends upon the discharge of the
sprinklers, which, in turn, depends upon the pressure. We find, there-
fore, a relation between the friction loss and the pressure. To clarify
these relations, we will start with the sprinkler-discharge equations 2
and 3, page 52, which show that the discharge of a sprinkler nozzle is
proportional to the square root of the pressure at the sprinkler.
Expressed mathematically,
q = KVP (16)
where q is the discharge of the sprinkler, K is a proportionality factor
depending upon the size of the sprinkler nozzles, and P is the pressure
at the sprinkler.
72 University of California — Experiment Station
Since the pressure along the line will vary because of the friction loss,
the sprinklers will not all discharge the same amount of water as was
assumed for the purpose of estimating the friction loss. However, the
total discharge of all the sprinklers is also proportional to the square
root of the pressure at any point on the line. If we assume that the
exponent m = 2 (equations 9 to 15), it follows that the friction loss will
be directly proportional to the pressure at any point on the line. If, for
example, the pressure at the pump is doubled, the friction loss will be
doubled, and the remaining pressure at the distal end of the line will
also be doubled. The ratio of the pressure at any sprinkler to the pres-
sure at any other sprinkler will therefore remain constant. For con-
venience, the pressure ratio will be defined as the ratio of the pressure
at any sprinkler on the line to the pressure at the distal end of the
line, and the discharge ratio as the ratio of the discharge of any sprinkler
on the line to the discharge of the sprinkler at the distal end of the line.
Thus there are definite pressure and discharge ratios for each sprinkler
on the line ; these remain constant regardless of the pressure. The dis-
charge ratio is equal to the square root of the pressure ratio, thus
(17)
where q is the discharge of any sprinkler at which the pressure is P, and
q0 is the discharge of the last sprinkler at the distal end of the line where
the pressure is P0. For pressure ratios less than 1.5, an approximate
relation is
|=l+0.5(|-l). (18)
Or, in words, the relative variation in the discharge of sprinklers along
the line is about half of the relative variation in pressure. For example,
a 20 per cent variation in pressure along the line will result in approxi-
mately 10 per cent variation in discharge of the sprinklers if all have
the same nozzle sizes.
The average pressure along a sprinkler line, Pa, approximates the pres-
sure at the distal end, P0, plus one fourth the friction loss in the line, Pf ;
or expressed mathematically,
Pa = Po + 0.25P/(approx.) ]
or \ (19)
Pa = Po + 0.25 (Pn - P0) (approx.) j
where Pn is the pressure at the sprinkler nearest the pump. Also, the
Bul. 670] Irrigation by Sprinkling 73
average discharge of the sprinklers can be expressed in terms of the
discharge of the first and last sprinklers, by the equation
% = % + 0.25 (Qn - %) (approx.) (20)
or in terms of the pressure at the first and last sprinklers, by the equation
% = Q0 [l + 0.12 (j? - ljj (approx.) . (21)
If the discharge of any sprinkler, qly at a given pressure, Plt is known,
then the discharge of the same sprinkler at pressure P2, is given by the
equation
ff, = 8iJp (22)
and the average sprinkler discharge corresponding to a pressure P2 at
any point on the line can be expressed in terms of the average sprinkler
discharge at pressure P1 in a similar manner.
The total flow is the product of the average sprinkler discharge and the
number of sprinklers on the line. Thus
Q = Nqa = Nq0 [l + 0.12 (y - l) J
(23)
where Q is the total discharge, and N is the number of sprinklers.
When the sprinkler lines are so designed that the pressure ratio does
not exceed 1.2, the variation in discharge of the sprinklers will not exceed
10 per cent, and the average discharge of the sprinklers will not exceed
the minimum discharge by more than 2.4 per cent. Of course, variations
in elevation along the line will affect both the pressure and the discharge
of sprinklers.
The power requirement for pumping water for sprinkling depends
upon the total pumping head and upon the total discharge of the line.
The total head includes (1) the lift from the water surface in the well
or ditch from which the water is pumped, (2) the friction losses in the
suction pipe or hose, (3) the velocity head, and (4) the pressure at the
pump. Where water is pumped from ditches, the lift and friction losses
in the suction hose are small in comparison with the pressure head. For
approximate calculations the velocity head may be neglected.
The horsepower requirement, hp., is given by the relation
where Q is the discharge in gallons per minute, Pv is the total pumping
74 University of California — Experiment Station
head, including lift and friction losses, expressed as pressure, and E is
the efficiency of the pump, expressed decimally.
The following example illustrates the use of these equations. A line
of 4-inch portable sprinkler pipe has thirty sprinklers, spaced 40 feet
apart. The discharge capacity of each sprinkler is 15 gallons per minute
at 40 pounds' pressure. The suction lift and the friction loss in the
suction hose approximate 3 pounds per square inch. The pump is 40 feet
from the nearest sprinkler, and the line is level. Assume a pump efficiency
of 60 per cent. What will be the total discharge, the required pressure
at the pump, and the horsepower requirement, for a minimum pressure
at the distal end of the line of 40 pounds per square inch ?
From figure 26, the friction loss for thirty sprinklers discharging 15
gallons per minute would be about 12 pounds per square inch. This must
40
be multiplied by—- = 2 to obtain 24 pounds per square inch, the actual
loss. Since this calculation was based upon an average discharge of 15
gallons per minute, it corresponds to an average pressure of 40 pounds
per square inch. The pressure at the distal end would be, from equa-
tion 19,
24
P0 = Pa — 0.25 Pf = 40 — - — = 34 pounds per square inch.
If the pressure at the distal end is increased to 40 pounds per square
inch, as specified by the problem, the friction loss will increase in pro-
portion, or
40
Pf = — X 24 = 28 pounds per square jneh.
And the average pressure will be (equation 19)
28
Pa = 40 H = 47 pounds per square inch.
The pressure at the pump will then be
PP = Po + Pf = 40 + 28 = 68 pounds per square inch.
The average sprinkler discharge, from equation 22, will be
/47
qa = 15 a / — = 16.3 gallons per minute.
The total discharge will be
Q = 16.3 X 30 = 489 gallons per minute.
Bul. 670] Irrigation by Sprinkling 75
The total pumping head (neglecting velocity head, but adding the suc-
tion lift) expressed as pressure, is
68 + 3 = 71 pounds per square inch.
The horsepower requirement will be (equation 24)
489 X 71
hp. = = 33.7.
F 1,715X0.60
An appreciable saving in power could be effected by using a split-line
arrangement — that is, where the source (portable pumping plant) is at
the center of the line the friction loss would be much less, and the sprin-
kler discharge fairly uniform along the line. According to calculations
similar to those above, with two 600-foot lines, the total discharge would
be 455 gallons per minute, the friction loss 3.4 pounds per square inch,
the pressure at the pump 43.4 pounds per square inch, and the power
requirement 20.5 horsepower.
Summarizing these principles, we can state :
The pressure at any point on the line is directly proportional to the
pressure at any other point on the line, and a change in pressure at one
point will result in a proportionate change in pressure at any other point.
The friction loss is directly proportional to the pressure at any point
on the line, and a change in pressure at any point will result in a pro-
portionate change in the friction loss.
The discharge of any sprinkler, and the total discharge, is propor-
tional to the square root of the pressure ; a change in pressure at any
point on the line will result in a change in discharge proportional to the
square root of the change in pressure.
The power requirement for pumping, being proportional to the prod-
uct of the pressure and discharge, is proportional to the three-halves
power of the pressure ; a change in pressure at any point will result in a
change in power requirement that is proportional to the three-halves
power of the change in pressure.
DISTRIBUTION OF WATER FROM ROTATING SPRINKLERS
The purpose of a sprinkler is to distribute the water to the soil in the
form of a sprinkle or spray so that it can be absorbed without running
off. Preferably, the water should be distributed uniformly over the area.
Since nearly all rotating sprinklers cover circular areas, an absolutely
uniform application is not possible. The degree of uniformity obtainable
depends primarily upon the type of distribution pattern produced and
upon the spacing of sprinklers. Numerous other factors, such as wind,
pressure, and uniformity of rotation of the sprinkler, affect distribution.
76 University of California — Experiment Station
Many tests have been made to determine the uniformity of distribu-
tion. Manufacturers generally have facilities for testing sprinklers and
making adjustments. A series of tests on both American and German
sprinklers was conducted by Staebner.18 These tests were made by catch-
ing the water in receptacles placed at various distances from the sprin-
kler. After the sprinkler had been operated for a specified time, the
amount of water caught in each receptacle was measured to determine
the depth. For most of his tests, Staebner used 112 cans and rain gauges
distributed in a geometrical pattern over a square area, 100 feet on each
side. Extensive tests have also been made in Germany,19 using a large
number of receptacles placed over the entire area covered.
Some manufacturers and experimenters have apparently been striv-
ing for a sprinkler that distributes a uniform amount over a large por-
tion of the area covered, with a rather abrupt breaking off at the edges.
Others have given more consideration to the effect of overlap and have
tried to obtain a different pattern. Staebner judged the sprinklers tested
on their ability to distribute water so that the maximum depth was not
more than twice the minimum, except near the edges of the area covered ;
but he did not discuss the question of overlap nor of proper spacing for
such sprinklers. He states, "No matter how successfully they may dis-
tribute water over a circular area, they leave much to be desired, because
if circles just touch one another a considerable area is left unwatered,
and if they overlap a great amount of double coverage results." He fur-
ther concludes, "More uniform distribution over a large area can be
obtained with the overhead-pipe system (nozzle lines) than with any
other type of spray irrigation equipment now available."
Before 1932 slow-revolving sprinklers were used mostly for stationary
overhead orchard systems and for irrigating large parks, golf courses,
and the like. In such installations, the common practice is to place the
sprinklers as far apart as possible and still cover all the ground. Sprin-
klers are generally mounted in either a square or a triangular arrange-
ment, the distance between them being as much as 80 to 90 feet. In 1932,
when portable systems first came into general use, the sprinklers, cover-
ing 100 to 120 feet circles, were usually spaced 40 feet apart on the pipe
line, which was generally moved 50 to 60 feet for each setup. This pro-
vided more than a double overlap in both directions. With such close
spacing it was assumed that the resulting distribution would be fairly
uniform.
"Staebner, F. E. Tests of spray irrigation equipment. U. S. Dept. Agr. Cir. 195:
1-29. 1931.
19 Studiengesellschaft fur Feldberegnung. Die Feldberegnung. RKTL Schriften
heft 13:1-177, 1930; heft 30:1-176, 1932; heft 38:1-177, 1933. Paul Parey, Berlin,
Germany.
Bul. 670] Irrigation by Sprinkling 77
SPRINKLER TESTS AT DAVIS
To obtain definite information about the distribution of water under
such conditions, and especially to determine the effect of wind, speed of
rotation, and spacing of sprinklers upon the distribution, a series of
tests on commercial sprinklers was made at Davis. For the first 122 tests,
small rain gauges made from no. 2% tin cans were placed 10 feet apart
in each direction over the entire area covered by the sprinkler except on
the north-south and east-west axes, where they were 5 feet apart. For sub-
//' p/pe nvafer //ne &
J i'p'pe pressure //ne **
I ° °/ " ° o ° ° ° / °
. . o o . . . / . O . o " o " . ' o " I " . " /" . " o
. 0.0 i
o / /CanJ . » 0 o o 00/
..jjL 00.0 00 00 0.0 ooo.ooo.o.o.^:
« ^JprinMler o o o o o 000
'ZM-y////:
Ud
Pig. 27. — Arrangements of cans for sprinkler tests at Davis. Left, the layout
for tests 1 to 122 j and right, for tests 123 to 170.
sequent tests an additional can was placed in each square, one can being
provided for each 50 square feet. Figure 27 shows the arrangements.
Water was supplied from the domestic system under a pressure of
about 40 pounds per square inch. To increase this pressure when desired,
a booster pump was provided. A 1-inch calibrated water meter measured
the discharge from the sprinkler. There was a return pressure line from
the base of the sprinkler riser, and a calibrated pressure gauge was in-
stalled near the pump and water meter. By means of a valve at this point
it was possible to regulate and maintain at the sprinkler a constant pres-
sure of any desired amount up to the maximum available. The arrange-
ment proved highly satisfactory.
A standard Weather Bureau type four-cup anemometer was installed
about 10 feet above the ground near the pump house, and the wind veloc-
ities given are those obtained at this location. The humidity and air
temperature were determined with a sling psychrometer, readings being
taken at intervals of 5 to 10 minutes during the tests.
78
University of California — Experiment Station
About 130 tests on slow-revolving sprinklers have been made with these
facilities. Some additional work with these sprinklers was done at Para-
dise, California. Lawn-sprinkler heads and small whirling sprinklers
have also been studied. Some of the slow-revolving sprinklers are shown
in figure 28. For most of these tests the sprinkler was operated for one
hour. The water caught in the cans was measured to the nearest cubic
centimeter, equivalent to %0o inch in depth. All tests were plotted as in
V <
Fig. 28. — Some sprinklers of the slow-revolving type used on portable sys-
tems that were used in the tests for distribution of water. Some of these have
been replaced by newer models.
figure 29 showing contours representing points of equal depth, and cross
sections in both the north-south and east-west directions. From these
tests a few have been selected to illustrate typical patterns for some of
the sprinklers tested; and to illustrate the effect of wind, insufficient
pressure, rapid rate of rotation, and variations in the rate of rotation.
Typical Distribution Patterns for Favorable Conditions. — Figures
29 to 33 illustrate typical distribution patterns for slow-revolving sprin-
klers operating under favorable conditions. For all but one of these tests
the wind velocity averaged less than 3 miles per hour. The speed of rota-
tion did not exceed one revolution per minute, and the pressure was
ample for proper distribution. The essential data for each test are given
in the figures and legends accompanying them. For four of the five
patterns , the average diameter covered exceeds 120 feet. These distribu-
Bul. 670]
Irrigation by Sprinkling
79
0ept/>, inches
£. W. Sec £ '/on
SO JO 40
30 20 /o o /o to JO
D/sto/Jce fro/n spri/?A/er, feet
40 JO 60
Fig. 29.— Test 16. Sprinkler, B-3 ; nozzles, %2 and %G in.
Test data: pressure, 45 pounds per sq. in.; discharge, 19.7
gal. per min.; average rate of rotation, 0.7 r. p. m. Condi-
tions for test favorable.
SO JO 40 JO 20 /O O /O tO JO ¥0
P/stonce /ro/77 spr/nfc/er, feed
Fig. 30.— Test 90. Sprinkler, F-l ; nozzles, % and % in.
Test data : pressure, 40 pounds per sq. in. ; discharge 14.3
gal. per min.; average rate of rotation, 1.0 r. p. m. Con-
ditions for test favorable.
80
University of California — Experiment Station
tion patterns are typical of a large number obtained and illustrate the
uniformity of distribution obtainable with sprinklers operating under
favorable conditions — that is, adequate pressure, low wind velocities,
and proper speeds of rotation. Particular attention is called to figures 31
and 33, for which the cross sections are somewhat triangular. As will be
shown later, this type of pattern produces the most uniform distribution
DefM, /nc/>es
J
\
£.
VSe
ct/or
SO 40 SO 20 /O O /O 20 30 40 SO 60
£>/'s£a/7ce /ro/7? spr/rtft/er, /<?<?/
£ S W
0/rect/o/> of /na/'/r /7o*z/e
Fig. 31. — Test 132. Sprinkler, B-4; nozzles, %2 and Vs in.
Test data : pressure, 50 pounds per sq. in. ; discharge, 12.0 gal.
per min. ; average rate of rotation, 0.29 r. p. m. Conditions for
test favorable. Lower profile shows variations in average rate
of rotation for different positions of the main nozzle.
over a large area when the sprinklers are properly spaced. Figures 31
and 32 include a graphic record of the average rate of rotation as deter-
mined for each 30-degree angle of rotation. The variations in the rate
of rotation were determined for most of the tests subsequent to test
number 52. There was evidence that variations in rate of rotation were
largely responsible for uneven distribution.
Effect of Low Pressure on Distribution from Sprinklers. — To deter-
mine the effect of low pressure, several tests were made at pressures
Bul. 670]
Irrigation by Sprinkling
81
inadequate for proper distribution. Figures 34, 35, 36, and 37 illustrate
typical patterns at pressures of 20 pounds per square inch. The most
noticeable feature is the ring near the outside edge of the area covered,
where the depth of application was several times greater than at a dis-
tance of 10 to 20 feet from the sprinkler. This doughnut or ring-shaped
pattern resulting from low pressure is one that generally produces a
^- 60 SO 40 30 20 /O O /O 20 SO 40 SO 60
^ 0/stance /row s/>r//7/c/er, feet
/Pot at /on of jpr/n/c/er
£ S W
Direct /'or? of /na/'n nozz/e
Fig. 32.— Test 136. Sprinkler, C-l ; nozzles, %2 and % in.
Test data : pressure, 50 pounds per sq. in. ; discharge, 19.3
gal. per min. ; average rate of rotation, 0.22 r. p. m. Condi-
tions for test favorable.
very uneven distribution for all reasonable spacings of sprinklers. The
effect of spacing on the distribution is discussed later in more detail.
Another noticeable feature of the lower pressure is the smaller area
covered. The average diameter of the wetted area is 90 to 100 feet, as
compared with diameters of more than 120 feet for pressures of 40 to
50 pounds and otherwise similar conditions. Judging from several tests
on the same sprinkler at different pressures, the reduction in area covered
is roughly proportional to the reduction in discharge of the sprinkler
caused by the lower pressure, while the average actual rate of applica-
82
University of California — Experiment Station
Depih, //k/?ss
V ft
60 so 40 so eo /O O /O SO 30
D/s/a/?ce /ram spr/nA/er, feet
SO 60 70
Fig. 33.— Test 170. Sprinkler, G-3; nozzles, % and %2 in-
Test data : pressure, 40 pounds per sq. in. ; discharge, 19.3 gal.
per min. Conditions for test favorable.
Pe/>M, mc/res
fO 40 so zo /o o /o to so 40 so
£>/'si<7/?ce /ram s/>r/'/?A/ert /eet
Fig. 34. — Test 11. Sprinkler, B-l ; nozzles, %6 and
%2 in. Test data: pressure, 20 pounds per sq. in.;
discharge, 14.2 gal. per min.; average rate of rota-
tion, 0.6 r. p. m. Pressure inadequate for good distri-
bution.
Bul. 670]
Irrigation by Sprinkling
83
\£
IV. J
ecf/o/?
/
\
J
\
/
\
so je 40 jo
0/sfortce from spr/n/t/er, fee/
Fig. 35.— Test 31. Sprinkler, A-2 nozzles, % and
%6 in- Test data : pressure, 20 pounds per sq. in. ;
discharge, 9.6 gal. per min.; average rate of rota-
tion, 0.7 r. p. m. Pressure inadequate for good dis-
tribution.
£>epl/>, //?c/>es
Til
JO 20 /O O /O ZO SO fO
/)/s/ trace /re/n sfr/'/rA/er, Zee/
£ S
f>/rec//b/? of tf?a//7 /tozz/e
Fig. 36.— Test 102. Sprinkler, G-l ; ...
and %2 in- Test data : pressure, 20 pounds per sq.
in.; discharge, 16.2 gal. per min.; average
rate of rotation, 0.24 r. p. m. This sprinkler is
equipped with a deflector on the tail nozzle which
breaks up the jet and improves the pattern. Note
the uniform rate of rotation.
84
University of California — Experiment Station
t>e/>t/>, /nc/res
«4 55
k°
£. M Sect /on
>
jo to so 20 /o o /o eo jo 40 so
0/stance from spr/nfi/er, feet
Direction of /na/n /tozz/e
Fig. 37.— Test 140. Sprinkler, C-l; nozzles, %2
and % in. Test data: pressure, 20 pounds per sq.
in.; discharge, 12.0 gal. per min.; average rate of
rotation, 0.29 r. p. m. Pressure inadequate for good
distribution.
Pep t/?, /notes
Jo 20 /o o /o to jo <?o so
P/stonce fro/n spr/n/r/er, feet
Fig. 38. — Test 58. Sprinkler, B-3 ; nozzles, %2
and %6 in. Test data: pressure, 40 pounds per
sq. in. ; discharge, 18.6 gal. per min. This pattern
illustrates the effect of wind.
Bul. 670]
Irrigation by Sprinkling
85
tion over the area covered, in inches (depth) per hour does not vary
materially with a variation in pressure.
Effect of Wind on Distribution Patterns. — That wind exerts a major
influence on the distribution pattern is illustrated by figures 38 to 41.
Although the water is thrown somewhat farther in the leeward direction,
it is thrown not nearly so far in other directions, and the area covered
is appreciably reduced. There is generally a high concentration of water
& /O O /O ZO JO
D/startce /ro/77 jpr/'nt/er, feet
Fig. 39.— Test 88. Sprinkler F-l ; nozzles, 5/16 and % in.
Test data: pressure, 45 pounds per sq. in.; discharge, 21.9
gal. per min. ; average rate of rotation, 2.6 r. p. m. This
pattern illustrates the effect of wind.
near the sprinkler, especially in directions normal to the direction of
wind, and a deficiency in the leeward direction.
The wind direction indicated is only approximate and was determined
from the sprinkler pattern. For the first few tests the wind was observed
at frequent intervals in order to estimate the approximate average direc-
tion. When, however, the shape of the sprinkler pattern proved to be a
reliable integrated measure of the wind direction, observations for this
purpose were discontinued.
Although the patterns appear very uneven, the effect of wind on the
uniformity of distribution over a larger area, with sprinklers close enough
together to provide an adequate overlap, is less serious than unevenness
from other causes, such as variation in rate of rotation, because with
wind the local areas of high and low concentrations always occur at the
86
University of California — Experiment Station
same relative position with respect to the sprinklers and do not overlap
on themselves and produce an exaggerated effect.
Effect of High Speed of Rotation on Distribution Patterns. — Figures
42, 43, and 44 illustrate how a high speed of rotation affects distribution.
The most noticeable feature of these patterns is the large reduction in
the area covered — a much greater reduction than results from a lowering
£<?/>//', McAes
*<.ss
£Hf
Jec t/o/>
1
& so a o /o eo 30 <o so
D/sta/tce fro/r> spr/nA/er, feet
flotation of jprinA/er
t=tt
A/ £ S W W
D/rect/b/7 of /na/n /?ozz/e
Fig. 40.— Test 100, Sprinkler, G-l; nozzles, %6
and %2 in« Test data: pressure, 30 pounds per sq.
in.; discharge, 19.6 gal. per min.; average rate of
rotation, 0.20 r. p. m. This pattern illustrates the
effect of wind.
of pressure to any point for which reasonable distribution of water can
be obtained. This reduction in area results in a corresponding increase
in the actual rate of application, which exceeded 1 inch per hour in a
localized area for one of the tests (fig. 43). This may be compared with
an average rate of application of less than % inch per hour for the same
sprinkler when rotating slowly. Obviously, sprinklers must be spaced
much closer together, when rotating rapidly, to secure the same uni-
formity of distribution. In addition, with portable systems, the lines
must be moved more frequently.
Bul. 670]
Irrigation by Sprinkling
87
/
\,
f.W. Ject/on
,r~
J
~^
\
s
40 JO SO X? O /O SO JO 40
D/sio/xre from jpr/rA/er, feet
20 \ 1 1 1 — Potof/on of jpr/nA/er
£ S h/
f//rect/b/? of mo/'/? /tozz/e
Fig. 41.— Test 112. Sprinkler 1-1 ; nozzle, %6
in. Test data: pressure, 40 pounds per sq. in.;
discharge, 22C7 gal. per min. ; average rate of
rotation, 1.3 r. p. m. This pattern illustrates the
effect of wind.
flept/!, //)c/>es
43' 4J' &
40 JO go /O O /O SO JO 40 SO
f)/s&ooce fro/7? spr/aA/er, feet
Fig. 42. — Test 21. Sprinkler B-2 ; nozzles, %2
and %2 in. Test data: pressure, 40 pounds per
sq. in.; discharge, 17.5 gal. per min.; average
rate of rotation, 13.6 r. p. m. This pattern
illustrates the effect of high speed of rotation.
Note the reduction in area covered and the cor-
responding increase in rate of application.
88
University of California — Experiment Station
Depi/>, /ncAes
%.so
;
V
/
' — *>
U
\^
\
/
\
-*
/
£.
JKJt
*ct/a
, \
^
30 so /o o /o SO SO
0/sfo/?ce /ro/n s/>r//7A/er, feet
Fig. 43. — Test 118. Sprinkler, 1-1; nozzle, %2 in.
Test data : pressure, 46 pounds per sq. in. ; discharge,
22.1 gal. per min. ; average rate of rotation, 26.3
r. p. m. This pattern illustrates the effect of high
speed of rotation.
Dep£/>, /aches
/
£.tf. Sect/ov
i i i
V.
SO 20 /O O /O tO JO fO
0/sfo/?ce /ra/7? spr//rfr/ert feet
Fig. 44. — Test 149. Sprinkler, L-l ; nozzle, %
in. Test data : pressure, 50 pounds per sq. in. ;
discharge, 17.0 gal. per min.; average rate of
rotation, 51.0 r. p. m. This pattern illustrates the
effect of high speed of rotation.
Bul. 670] Irrigation by Sprinkling 89
Slow-revolving sprinklers are designed to be operated at speeds of
about one revolution per minute, and should never rotate faster than
three or four revolutions per minute. They are sometimes provided with
a device for regulating the speed. Sometimes, however, they rotate very
rapidly ; in one instance a count showed a sprinkler on a portable system
to be averaging 90 revolutions per minute. Rotation speeds of 20 revolu-
tions per minute are common on many systems. Some sprinklers are
so designed that the speed of rotation can be adjusted only within certain
limits, or not at all, and high speeds are impossible. Excessive speeds
increase the wear on the bearings and spindles, which in turn may cause
the sprinkler to rotate unevenly.
SO* o/je eomp/ete revo/e/t/o/7
I =4
" — -y~ t N — M — * — " — "— » — " — h ' \/m/'/?i//e I
*dbt/6/e /narA-sna//? />ozz/e po//7//'/?q- /tort/? *"
Fig. 45. — Kecorder chart showing how variation in rate of rotation of
sprinklers was determined. The pen makes a mark for each 30 degrees of rota-
tion, a double mark for each revolution of sprinkler. Uneven spacing of marks
shows variation in rate of rotation.
Effect of Variation in Bate of Rotation on Distribution Patterns. —
Before very many tests had been made, it was apparent that some of the
unevenness in distribution was caused by the sprinkler's rotating at a
variable rate through each revolution. With rates less than one revolu-
tion per minute, this variation is not noticeable to the eye; but by timing
the sprinklers through consecutive quadrants with a stop watch, it was
found that there were appreciable variations in the average rate for the
different quadrants. The rate was always slow or fast (as the case might
be) for the same position of the sprinkler, that is, when a nozzle was
pointing in the same direction, so that more water was being thrown in
one direction from the sprinkler than in another. To investigate this
point thoroughly, an anemometer recorder was converted into a rota-
tion recorder by changing the gear ratio so that the drum turned at a
peripheral speed of 1 inch in 50 seconds. A commutator arrangement
was attached to the sprinkler so that an electrical circuit was shorted
for each 30 degrees, or 60 degrees, of rotation as desired. For speeds ex-
ceeding about 1 revolution per minute, 60-degree intervals were used;
for speeds of less than this amount, 30-degree intervals. This recorder
operated successfully only when the sprinkler rotated at a speed of less
than three revolutions per minute. The data were tabulated by measur-
ing the number of seconds for each point of contact on the chart with
a scale having 50 graduations per inch. The recorder was generally
90
University of California — Experiment Station
operated 30 to 45 minutes during a test ; and the data were tabulated for
this period, or up to a maximum of 30 or 40 revolutions. The average
rate of rotation for each 30- or 60-degree angle was calculated and
plotted graphically as illustrated.
These determinations were made for most of the tests subsequent to
test no. 52, except those for which the rate of rotation was too high to
0epl/)t /fie/res
60 SO 40 SO ZO /O O /O ZO JO 40 SO GO
£/sfo/Jce fro/n s/>r//7A/er, feet
£ S W
0/rect/o/? of /now nozz/e
Fig. 46. — Test 103. Sprinkler, L-l ; nozzles, %6 and !^4 in.
Test data : pressure, 40 pounds per sq. in. ; discharge, 22.5 gal.
per min. ; average rate of rotation, 0.68 r. p. m. This pattern
illustrates how a large variation in rate of rotation affects the
distribution of water. Both nozzles of the sprinkler were point-
ing in approximately the same direction.
be determined. The apparatus for these tests and the method of analyzing
the data proved very satisfactory, presenting definite evidence that
uneven distribution in many tests was due largely to variations in the
rate of rotation.
Figure 45 is a typical record of the rate of rotation as obtained with
the recorder. The variation in rate is indicated by the difference in
spacing between the marks on the lines.
Figures 46 and 47 illustrate how extreme variations in the rate of
Bul. 670]
Irrigation by Sprinkling
91
rotation affect distribution. For figure 46, both nozzles of the sprinkler
were discharging in approximately the same direction; and the local
areas of high concentration to the southwest of the sprinkler are due to
the very slow rate of rotation of the sprinkler when discharging in that
direction, as indicated by the graphic record of the rotation. For figure
47 the two nozzles were discharging in approximately opposite direc-
Dep£/!t //tehes
<0 JO £0 /O O /O 20 JO 40 JO 60
0/st<7/?ce /row jpr/Wfr/er, feet
£ S
0/rect/b/? of m<7//? /?otz/e
Fig. 47.— Test 115. Sprinkler, 1-1; nozzle, %2 in. Test
data: pressure, 50 pounds per sq. in.; discharge, 22.7 gal.
per min. ; average rate of rotation, 0.80 r. p. m. This pat-
tern illustrates the effect of a variable rate of rotation. The
two nozzles on this sprinkler were pointing in opposite
directions; the high rate of application to the northeast
of the sprinkler is due to the tail nozzle, while that to the
southeast is due to the main nozzle.
tions ; the high concentration to the northeast of the sprinkler is from
the tail nozzle, whereas that in the southwest direction is from the main
nozzle. That the lowest rate of rotation occurred for both tests (different
sprinklers) when the main nozzle was pointing in a southwesterly direc-
tion was apparently accidental. Other tests indicated that the positions
of maximum and minimum rates are apt to occur in any direction and
that the variation is not due to wind, as some sprinkler users suppose.
92
University of California — Experiment Station
Judging from tests on several different makes of sprinklers during
strong winds — in one instance averaging more than 25 miles per hour —
the positions of high and low speed bore no relation to wind direction,
but could be altered at will by rotating the base of the sprinkler. One
sprinkler was operated for a given period; then the base was oriented
180 degrees, and the test repeated. This caused the positions of high and
low rates of rotation also to be oriented about 180 degrees, though the
direction of the wind remained approximately the same (fig. 48).
Fig. 48. — Variation in rate of rotation of
sprinkler illustrated by plotting the rate as a
radius. The similarity in shape and opposite
orientation of the two diagrams indicates that
the variation in rate of rotation was not caused
by the wind.
Variations in rate of rotation is undoubtedly due to a variation in
the friction on the bearing when the sprinkler is in different positions.
Since the driving force is relatively constant, a variation in speed
results when the frictional resistance varies. When operating under
adequate pressure the frictional resistance for most sprinklers is suffi-
cient so that the unbalanced torque, due to the wind reaction on the
sprinkler, has little effect on the speed of rotation. Whether the varia-
tion in frictional resistance results from a lack of precision in manuf ac-
Bul. 670]
Irrigation by Sprinkling
93
ture, from variations in thickness of the leather washer in the bearing,
or from other causes was not determined. All sprinklers tested were
new and unworn. Field observations indicate that worn sprinklers fre-
quently vary more in speed of rotation than new sprinklers and that
there is a great difference in the performance of sprinklers of the same
kind on the same line.
DESIRABLE TYPES OF DISTRIBUTION PATTERNS AND
PROPER SPACING OF SPRINKLERS
The distribution patterns in the foregoing figures immediately suggest
two questions : What is the most desirable type of distribution pattern ?
Air /no/e/neni yar/ab/e
0.45 m.p./?.
U^lO.Z-0.4 metes &
E23 0.4 -0.6 •
1ZZA0-6-0.8 *
■H over i.0 '
Fig. 49. — Kesulting distribution of water when sprinkler patterns are overlapped
to correspond to a spacing of 40 by 50 feet. A, Sprinkler pattern for test no. 16 (fig.
29) illustrates favorable conditions (pressure, 45 pounds per sq. in.). B, Pattern for
test no. 11 (fig. 34) illustrates inadequate pressure (20 pounds per sq. in.).
What is the proper spacing of sprinklers for the most uniform distribu-
tion ? All the sprinkler tests and many geometrical patterns were ana-
lyzed for the purpose of answering these basic queries.
To determine the uniformity of distribution for a group of sprinklers,
some of the patterns were overlapped at a spacing of 40 feet in one direc-
tion by 50 feet in the other (a common arrangement for portable systems)
and the resulting depth of application was calculated for points spaced
10 feet apart both ways. The application from all sprinklers affecting
the net area was included. Figure 49 shows the results for two tests
previously illustrated. For test no. 16, with the sprinkler operating
under favorable conditions, the resulting distribution is remarkably
uniform; but for test no. 11 (inadequate pressure) the variation is
from about 0.2 inch to more than 1.0. This analysis, though it serves
94 University of California — Experiment Station
to illustrate the distribution obtained for specific tests and spacings,
does not answer the principal questions involved.
A Method of Analyzing Sprinkler Tests for Uniformity of Distribu-
tion.— To compare sprinkler patterns and to determine how various
spacings affect the resulting distribution of water, one needs a numerical
expression to serve as an index of the uniformity secured. For this pur-
pose an expression called the uniformity coefficient (Cu) was adopted.
The uniformity coefficient expressed as a percentage is defined by the
equation , .
Cu= 100(1.0- — ) (25)
\ mn)
in which x is the deviation of individual observations from the mean
value m, and n is the number of observations. An absolutely uniform ap-
plication is then represented by a uniformity coefficient of 100 per cent ;
a less uniform application, by some lower percentage.
When the intensity of application at any number of equally spaced
points over the entire area covered by a sprinkler is determined, the
uniformity coefficient can be computed for any spacing (in either direc-
tion) that is a multiple of the spacing of the points of observation. Thus
a complete analysis of one sprinkler pattern to determine the best
spacing and the resulting uniformity of application involves numerous
computations.
Uniformity coefficients have been determined both for actual sprinkler
patterns and for various geometric patterns. The depths of application
at uniformly spaced points over the net area covered by a sprinkler are
first determined. Sufficient points are used so that the depth at any
particular point may be considered the mean for the unit area repre-
sented by that point. For the actual sprinkler patterns, the amount of
water caught in each of the cans spaced 5 or 10 feet apart in parallel
rows is taken as an individual observation. The pattern is then over-
lapped on itself to correspond to any desired spacing, and the total
amount for each point within the net area covered by one sprinkler is
determined and tabulated. The mean depth of application is next de-
termined, and the deviation from the mean at each point is calculated.
About half of the points will have applications of more, and half of less,
than the mean. These deviations are then totaled, and the uniformity co-
efficient is computed from equation 25. For each sprinkler pattern, a
different value of the uniformity coefficient is obtained for each spacing ;
and since the spacing may be different in the two directions, many
tedious calculations are required to analyze one sprinkler pattern com-
pletely and determine what spacing will give the best results and how
uniform the distribution will be.
Bul. 670] Irrigation by Sprinkling 95
A short-cut method of determining the optimum spacings has been
employed in analyzing all the sprinkler tests. This method is equivalent
to spacing the sprinklers closely along the pipe line and then determin-
ing the uniformity coefficients for different spacings between lines. ($x
is used to denote the spacing between sprinklers along the line, and S2
the spacing between lines.)
When sprinklers are close together (say, S1 = 10 feet), a strip of
ground will be wet so that there will be little variation along lines
drawn parallel with the line of sprinklers. The profile of water concen-
tration across the wetted strip along any line drawn at a right angle to
the pipe line can be determined by overlapping a sprinkler pattern upon
itself corresponding to the designated sprinkler spacing (&J. This is
done by summing up the water caught in the cans in each of the parallel
rows. The extra cans on the two diameters are omitted so that all cans
used are equally spaced over the area. The tabulated sums are then
combined, corresponding to various spacings (S2) between lines, and
the uniformity coefficient calculated. The coefficients thus determined
represent a measure of uniformity in only one direction, not a measure
of the uniformity for the net area covered by sprinklers spaced normal
distances along the line. Since the actual sprinkler patterns are not
symmetrical, because of wind and other influences, the uniformity co-
efficients were determined for two directions normal to each other, and
the mean value for each spacing was used. These analyses indicate the
optimum spacings, which apply both along the lines and between lines.
Distribution for Geometrical Patterns. — Before discussing the results
of the analyses of the actual sprinkler tests, we may well consider the
distribution for certain geometrical patterns. If a sprinkler is rotated
at a uniform speed in perfectly still air, the resulting pattern will be
symmetrical about the center. Several symmetrical patterns represent-
ing different geometric shapes have been analyzed to determine the
uniformity of distribution for various spacings. Six of these patterns,
some being typical of actual sprinkler patterns, are shown in figure 50,
together with the uniformity coefficients for different values of S2 when
S1 equals 5 per cent of the diameter covered. The distribution for pat-
tern B is nearly uniform for all values of S2 up to 55 per cent of the
diameter. Patterns A and C give fairly uniform applications for all
spacings up to 65 per cent of the diameter; beyond this the uniformity
drops off rapidly. Patterns D and E produce a fairly uniform distribu-
tion for spacings of 75 and 80 per cent; but for spacings of 45 to 70
per cent there is an appreciable variation. Pattern F results in poor
distribution for spacings of 50 to nearly 80 per cent, with only fair
uniformity for spacings between 80 and 85 per cent of the diameter.
96
University op California — Experiment Station
The uniformity coefficients for various spacings in both directions
for patterns B and E, shown in figure 50, are given in table 11. This
table illustrates how the uniformity coefficients vary with Sx as well as
with 82. For pattern B the uniformity coefficients remain constant as 8X
is increased from 5 up to about 60 per cent of the diameter. For pattern
E, however, with the spacing 8X equal to 5 per cent of the diameter,
(0.05 D), the uniformity coefficient drops from 97 when S2 is 0.4 D, to
20
30 40 50 60 70 0O
Spacing be £ ween //nes, 5zt % of d/a/neter
Fig. 50. — Geometrical sprinkler patterns and uniformity coefficients for different
spacings of lines with sprinklers spaced closely along the lines. The higher the uni-
formity coefficient, d, the more uniform the application.
80 when S2 is 0.6 D, and back up to 96 when S2 is 0.8 D. For a spacing
between lines, S2y of 80 per cent of the diameter, the uniformity coeffi-
cient drops from 96 when 8X is 0.05 D to 68 when 8t is 0.6 D, then rises
to 74 when Sx is 0.8 D for a square arrangement, and to 83 when 8X is
0.8 D for a triangular arrangement. In general, these and other patterns
studied in detail indicate that the analyses for S± equals 0.05 D or 0.1 D
(the short-cut method) are reliable for determining optimum spacings,
and that fairly uniform distribution results when 8X does not exceed
half the optimum values of 82. If, for example, a spacing of 0.05 D by
0.8 D shows a high uniformity coefficient, a spacing of 0.4 D by 0.8 D is
likely to give good results. When a fairly close spacing between pipe
lines is permissible, as it sometimes is on portable systems, excellent re-
sults can be obtained with patterns similar to D and E when the spacing
Bul. 670]
Irrigation by Sprinkling
97
between lines, S2, is also half the maximum optimum spacing as indicated
by this method of analysis. This spacing provides in effect, a double over-
lap in both directions.
Desirable Patterns for Square and Equilateral-Triangle Arrange-
ments of Sprinklers. — On stationary sprinkling systems, for economic
TABLE 11
Uniformity Coefficients for Various Arrangements and
Spacings of Geometrical Patterns B and E*
Spacing between lines, S2, in per cent of the diameter
,D
Spacing along line, Si,
in per cent of diameter, D
40
per cent
50
per cent
60
per cent
70
per cent
80
per cent
Pattern B
5
99
99
98
97
98
98
97
96
96
93
93
93
93
93
91
92
80
80
80
SO
80
80
80
77
78
67
20
67
30
67
40
67
50
67
60
67
60 Tf
67
70
65
70 T
65
80
58
80 T
60
Pattern E
5
97
96
93
96
84
84
83
83
76
80
80
80
77
77
75
67
86
86
85
83
82
71
66
66
65
96
20
94
30
93
40
88
50
81
60
68
60 T
70
70
67
70 T
73
80
74
80 T
83
* Shapes of patterns are shown in figure 50.
t T denotes a triangular arrangement of sprinklers; all others are rectangular.
reasons, sprinklers are usually spaced as far apart as possible in either
a square or an equilateral triangle arrangement. One should know what
patterns will give most uniform application, and how far apart sprin-
klers can be placed with satisfactory results. To answer these questions,
a set of patterns that give as nearly uniform applications as possible for
different spacings have been developed.
As previously shown, a conical pattern (pattern B in fig. 50) with a
triangular cross section gives nearly uniform applications for spacings
98
University of California — Experiment Station
up to about 55 or 60 per cent of the diameter. Slightly modified patterns
of this general shape result in even greater uniformity for similar spac-
ings. Figure 51 shows a series of curves illustrating half cross sections
of patterns, as determined by trial and error methods, that give maxi-
mum uniformity coefficients for spacings of 50 to 75 per cent of the
diameter when the sprinklers are arranged in a square. When the spacing
exceeds 71 per cent of the diameter, the application at the center of the
square between four sprinklers will be zero regardless of the shape of the
20 30 40 £0 60 70
P/sfo/rce /row spr/rtA/er, % of rad/'i/s covered
Fig. 51.—
■Half cross sections of patterns that give maximum uniformity coefficients
for different spacings of sprinklers when arranged in a square.
pattern. A spacing of 71 per cent of the diameter is therefore the maxi-
mum possible if all the area is to be covered.
Detailed analyses of three of these patterns, G, I, and K, have been
made for square spacings (S1=S2) from 35 to 75 per cent of the diameter.
Table 12 gives the uniformity coefficients for these patterns both for
square spacing and for a very close spacing along the line (8± = 0.05 D)
for various spacings between lines. Data in table 12 are shown graphi-
cally in figure 52 which illustrates the relation between the uniformity
coefficients obtained for square spacing and for close spacing along the
line. In addition, the uniformity coefficients are shown for pattern K
for $x = 0.4 D. For pattern G the curves are similar, the close spacing
showing slightly higher uniformity coefficients for all spacings between
lines. For pattern I there is somewhat more difference between the curves,
60 per cent spacing between lines (S2 = 0.6 D) giving the highest uni-
formity coefficients for close spacing, whereas for the square arrange-
Bul. 670]
Irrigation by Sprinkling
99
ment a 55 per cent spacing (S1 = S2 = 0.55 D) gives the best results.
For pattern K, the most desirable pattern for a spacing of 70 per cent
(St = S2 = 0.7 D), there is still more difference. When S1 = 0.05D
much better results are obtained for values of S2 between 60 to 70 per
cent of the diameter than for spacings of 45 to 60 per cent, whereas for
a square arrangement the uniformity coefficients are fairly constant
TABLE 12
Uniformity Coefficients for Patterns G, I, and K*
Spacing be-
tween lines,
S2, in per cent
of diameter
Net area,
in per cent of
circular area
covered
Uniformity coefficients, in per cent,
for the patterns:
For square arrangement of sprinklers (Si = S?)
35
15.4
99
97
97
40
20.1
96
96
95
45
25.4
97
92
87
50
31.8
99 1
98
86
55
38.5
93
96
87
60
45.6
85
93
87
65
53.8
78
87
85
70
62.4
69
80
86
75
71.6
72
80
For close spacing along the line (Si = 0.05Z))
50
3.2
100
94
89
55
3.5
95
97
90
60
3.8
89
98
93
65
4.1
82
92
97
70
4.5
74
85
94
75
4.8
78
87
80
5.1
80
* Shapes of patterns are shown in figure 51.
t Italicized figures correspond to the spacing for which the pattern is
designed.
for all spacings between 45 and 70 per cent of the diameter. The curve
for a 40 per cent spacing along a line (Sx = 0.4 D) is very similar to
that for close spacing (#1 = 0.05 D) indicating that for customary
spacings of sprinklers on portable systems, the results of analyses for
close spacings along the line show what spacings between lines, S2, will
give the best distribution.
For an equilateral-triangle arrangement of sprinklers, patterns of a
slightly different shape give better distribution. Figure 53 shows half
cross sections of nine patterns that give the highest uniformity coeffi-
cients for spacings between sprinklers, 8lf of 50 to 90 per cent of the
diameter. For this arrangement the spacing between parallel rows of
sprinklers, S2, is only 86.6 per cent of the spacing between sprinklers.
100
University of California — Experiment Station
45 SO 55 SO 65
Spac/'/ig tettveefl ///res, J^, Z of rf/omete r
Fig. 52. — Uniformity coefficients for patterns G, I, and K (fig. 51) for close spacing
along the line and for square spacing of sprinklers.
Table 13 gives the uniformity coefficients for four of these patterns —
M, 0, Q, and S for both a triangle arrangement and for close spacing
along the line. The uniformity coefficients for both arrangements are
compared in figure 54 for patterns M and S. There is a striking similarity
in the two curves for both patterns. Uniformity coefficients for the close
k /oo
x 30
70 —
X
^ so
\
*■*
0
M
so
N
55
O
60
P
65
0
70
R
75
S
80
r
85
u
SO
20 50 40
D/sta/?ce fro/7? jpr/nk/er,
50 60 70
7c of rad/t/s covered
Fig. 53. — Half cross sections of patterns that give highest uniformity coefficients for
equilateral-triangle arrangement of sprinklers with different spacings.
Bul. 670]
Irrigation by Sprinkling
101
spacing (#! = 0.052)), though somewhat higher than for the equilat-
eral-triangle arrangement with 8t = 0.866 2), do, however, indicate the
spacings between lines, S2f for triangular arrangements that give the
best distribution.
TABLE 13
Uniformity Coefficients for Patterns M, O, Q, and S*
Spacing be-
tween sprink-
lers, Si, in
Spacing be-
tween lines,
S2, in per cent
of diameter
Net area,
in per cent
Uniformity coefficients, in per cent,
for the patterns :
per cent of
diameter
of circular
area covered
M
0
Q
s
For equilateral-triangle arrangement of sprinklers (Si =
0.866 D)
30
26.0
9.9
99
99
99
99
35
30.3
13.5
97
98
98
94
40
34.7
17.6
99
98
99
95
45
39.0
22.3
99
98
94
97
50
43.3
27.6
90t
96
89
89
55
47.7
33.4
97
96
87
82
60
52.0
39.7
91
98
88
78
65
56.3
46.6
82
94
92
78
70
60.7
54.0
72
87
97
82
75
65.0
62.0
77
90
89
80
69.3
70.6
67
79
94
85
73.7
79.7
68
86
90
78.0
89.4
73
For close spacing along the line (Si = 0.05 D)
5
40
2.5
99
98
95
97
5
45
2.9
99
97
92
91
5
50
3.2
96
98
92
87
5
55
3.5
90
98
94
87
5
60
3.8
84
93
99
90
5
65
4.1
77
86
94
94
5
70
4.5
69
78
87
96
5
75
4.8
62
72
80
90
5
80
5.1
65
72
83
5
85
5.4
66
77
5
90
5.7
69
* Shapes of patterns are shown in figure 53.
t Italicized figures correspond to the spacing for which the pattern is designed.
Judging from these studies, a more uniform application can be ob-
tained with a triangular arrangement of sprinklers than with a square
arrangement. This is particularly true for the wider spacings. A com-
parison of the uniformity of distribution obtainable with both arrange-
ments is shown in figure 55. The points plotted correspond to the pattern
giving the highest uniformity coefficient for each spacing. To secure
any added benefit from the triangular arrangement, however, the sprin-
klers must be more accurately spaced than for the square arrangement.
A comparison of patterns K and S will illustrate this point. Pattern S
102
University of California — Experiment Station
4S JO S5 60 65
Spac/na t>elu/eef! ///?eslS2) % of a'/ameter
Fig. 54. — Uniformity coefficients for patterns M and S (fig. 53) both for close
spacing along the line and for equilateral-triangle spacing of sprinklers. Note par-
ticularly that although pattern S gives a fairly high uniformity coefficient for a
spacing of 70 per cent of the diameter covered by the sprinkler, the uniformity co-
efficient is fairly low for spacings between 40 and 65 per cent of the diameter.
has a uniformity coefficient of 94 for a triangular arrangement with
St = 0.8 Z), which corresponds to a net area of 70.1 per cent of the cir-
cular area covered by the sprinkler. Pattern K has a uniformity coeffi-
cient of only 86 per cent for a square spacing of 0.7 D, which corresponds
•5j so
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JO 40 JO 60 70
Net area, % of area a/" c/rc/e caverea"
Fig. 55. — Comparison of highest uniformity coefficients for different patterns for
square and equilateral -triangle arrangements of sprinklers. Higher uniformity coeffi-
cients can be obtained for the triangular arrangement when the sprinklers are cor-
rectly spaced.
Bul. 670] Irrigation by Sprinkling 103
to a net area of 62.4 per cent. When the spacing between sprinklers for
pattern S is changed to 0.65 D, the uniformity coefficient drops to 78 per
cent, whereas for pattern K, the uniformity coefficient remains above 85
per cent for all square spacings closer than 70 per cent.
UNIFORMITY OF DISTRIBUTION AND EFFECT OF SPACING
ON ACTUAL SPRINKLER PATTERNS
A study of geometrical patterns to determine the uniformity of dis-
tribution is not entirely adequate, since actual sprinkler patterns are
never symmetrical in shape because of wind and sometimes because of
variations in the speed of rotation. To determine the uniformity of dis-
tribution corresponding to the actual sprinkler patterns, all of the tests
suitable for this purpose were analyzed. The uniformity coefficients,
corresponding to a spacing, 8lt of 10 feet along the line and spacings
between lines, 82, of 20 to 100 feet, were computed for both the east-west
and north-south directions. Table 14 gives the mean uniformity coeffi-
cient for each spacing for the two directions, together with other per-
tinent data.
These data are summarized in table 15, where several tests on a given
sprinkler under similar conditions of wind, pressure, and rate of rota-
tion are averaged.
Table 16 gives the uniformity coefficients for various spacings along
the line and between lines for sprinkler patterns illustrated in figures
29, 31, 33, 34, 39, and 43.
This table brings out several important facts regarding the uniformity
of application of water by sprinklers. A comparison of the uniformity
coefficients for tests 16, 132, and 170 for specific spacings will illustrate
the effect of shape of pattern. For test 16, the optimum spacing 82 is 80
to 90 feet; for spacings 60 and 70 feet, there is excessive overlap, and
the uniformity coefficient is somewhat lower. This characteristic is not
evident for tests 132 and 170 (patterns triangular in shape) but the
optimum spacing is reduced to about 60 feet. The coefficients for test 11
illustrate the effect of inadequate pressure (20 pounds per square inch) .
For this pattern a fairly good distribution is obtained with a spacing
of 10 by 80 feet, but when the spacing 8t is 40 feet, the results are very
poor for all spacings S2, the coefficient being only 57 for a spacing of 40
by 80 feet. For test 170, the uniformity coefficients remain practically
constant for any spacing 82 when the spacing #, is increased from 10 feet
up to 60 feet. The uniformity coefficients for test 88 (fig. 39), showing
the performance of a sprinkler with a wind velocity of 10.7 miles per
hour and for test 118 (fig. 43) where the sprinkler rotated rapidly
(26.3 revolutions per minute), indicate to what extent the effective area
104 University of California — Experiment Station
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Bul. 670]
Irrigation by Sprinkling
109
TABLE 16
Uniformity Coefficients for Actual Sprinkler Patterns
Spacing
along
Uniformity coefficients, in per cent, for various spacings between lines, 52
line, <Si,
feet
40 feet
50 feet
60 feet
70 feet
80 feet
90 feet
100 feet
Pattern for test no. 11 (fig. 34)
10...
20...
30...
40...
60...
60 T*
10...
20...
30...
40...
60...
60 T.
80...
80 T.
10...
20...
30...
40...
60...
60 T.
10...
20...
30...
40...
60...
60 T.
10...
20...
30..,
40..
60...
60 T
80...
80 T
10..
20..
30...
40...
60..
60 T
80..
80 T
76
Pattern for test no. 16 (fig. 29)
89
88
Pattern for test no. 88 (fig. 39)
Pattern for test no. 118 (fig. 43)
Pattern for test no. 132 (fig. 31)
90
97
87
Pattern for test no. 170 (fig. 33)
79
T denotes a triangular arrangement of sprinklers; others are rectangular.
110 University of California — Experiment Station
covered is reduced when operating conditions are unfavorable. For both
of these tests, the size of the sprinkler nozzles and other factors were
comparable with tests 16 and 170. (For specific data on these tests see
table 14).
EVAPORATION LOSSES
A question frequently asked concerning sprinkling is, How much
water is lost by evaporation when water is sprayed into the air ? Gener-
ally it has been assumed that the loss directly from the spray may be
appreciable, especially on warm, dry days, and when the wind is blowing.
In addition to the loss from the spray there are direct evaporation losses
from wet surfaces during and following every application of water.
EVAPORATION FROM THE SPRAY
There has been little published from which one might obtain infor-
mation on the evaporation from water drops moving rapidly through
the air. To compare sprinkling with other methods of irrigation, one
must separate these losses from those which occur subsequently from the
wet soil, and which are common to all irrigation methods. In order to
obtain some information on this subject, the sprinkler tests for distribu-
tion of water were planned so that the total amount of water applied to
the entire area could be calculated from the amounts caught in the cans.
This was one of the principal reasons for spacing the cans uniformly
over the entire area.
Direct Measurement of Evaporation Losses. — Before evaporation
losses could be ascertained it was first necessary to know how accurately
the total amount of water falling on the area could be determined by this
method. To do this, a series of tests were made in the early morning
before sunrise when the relative humidity is highest and the air tem-
perature lowest. It was reasoned that the evaporation at this time of day
would be very low, possibly negligible. The calculated loss, or difference
in the amount of water falling on the area, and that discharged from
the sprinklers, should then be low. For fifteen early-morning tests, where
the relative humidity exceeded 75 per cent, the calculated loss was
3.9 ± 0.35 per cent. This loss included that represented by the water
which would cling to the inside of the cans when emptied. By carefully
weighing cans dry and after pouring water from them, this part of the
loss was found to average about 1 cubic centimeter, or about 3 per cent
of the average amount caught. Judging from these early-morning tests,
the total amount of water falling on the area can be determined with
reasonable accuracy from the amounts caught in cans uniformly spaced
over the area.
Bul. 670]
Irrigation by Sprinkling
111
The calculated losses for afternoon tests varied from less than 10 per
cent to a maximum of 42 per cent for one test when the air temperature
averaged 105 °F and the relative humidity about 15 per cent. These
losses did not show a very high correlation with the evaporation from
atmometers, or with such factors as relative humidity and vapor pressure
deficit, but sunshine appeared to have an important effect. Further study
showed definitely that a large part of the calculated loss could be ac-
counted for by evaporation from the cans. To reduce this loss, funnels
were soldered into the cans. Comparative tests were made by placing 50
^ 30
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20 30 40 SO 60 70
Approximate average re/ai/'ve /iv/nid/fy, per ce/?t
SO
Fig. 56. — Evaporation loss calculated from sprinkler tests. This loss includes that
from the spray, that from the cans during the test and period of measurement, and
that represented by the amount of water clinging to the cans when they are emptied.
cubic centimeters of water in each of five cans with, and five cans without,
funnels. After 3 hours the average loss from the cans with funnels was
only 0.75 cubic centimeter per hour, or about 10 per cent of that from the
open cans.
The first 122 sprinkler tests were made with open cans spaced 10 feet
apart in each direction. For subsequent tests, the cans with funnels were
spaced 7.07 feet apart in diagonal rows, providing one can to each 50
square feet (fig. 27).
The use of the cans with funnels did not, however, materially change
the calculated losses as indicated by figure 56, which shows the loss
plotted against the approximate average relative humidity. The reason,
probably, is the evaporation of water from the wet surface of the funnel.
No way was found to eliminate this error. The measured losses should,
therefore, be considered not as evaporation losses from the spray alone,
but as the combined losses from the spray and from the cans during the
112 University of California — Experiment Station
test and the period of measurement following the test, which averaged
about 30 minutes.
Indirect Method of Estimating Evaporation Loss from the Spray. —
The evaporation loss from the spray may be determined approximately
from thermodynamic principles. Evaporation of water requires heat.
The number of calories necessary to evaporate one gram of water, called
the latent heat of evaporation, varies from 539.6 for water at 100° C
(boiling point) to 595.9 for water at 0° C (freezing point). It is about
585 for water at 68° F. Three sources of heat are available for evap-
orating water from a spray : (1) heat from the water, (2) heat absorbed
from the air, and (3) radiant heat, principally from the sun. If all of
the heat came from the water, it would require a temperature drop of
about 10.5° F to evaporate 1 per cent of the water.
When the water is cooler than the air, as normally in the daytime, the
water will absorb heat from the air, and the drop in temperature will be
less than 10.5° F for 1 per cent loss. Absorption of radiant heat will also
increase the evaporation for the same temperature change. When the
initial water temperature is the same as the wet-bulb temperature
of the air, an equilibrium condition exists; all the heat required for
evaporation comes from the air, and the water remains at a constant
temperature. When the initial temperature of the water is lower than
the wet-bulb temperature, it will increase even though some evaporation
still takes place. The evaporation would be zero, however, if the water
temperature was at the dew point; and if it were lower than this, con-
densation would occur, and there would be a gain rather than a loss of
water.
Neglecting the radiant heat and considering only that from the air
and water, an equation has been derived from which the evaporation
loss can be determined for any given change in the temperature of the
water, provided the air temperature and humidity are known. This ap-
proximate expression for the evaporation loss from the spray is
= lQQCAt r pw-pa i
r [P» -Pa- 0.00037 B(ta - O J V ;
where E is the loss of water from the spray expressed as a percentage of
the amount discharged; C is the specific heat of water, calories per
gram per degree Fahrenheit ; r is the heat of vaporization, calories per
gram ; At is the drop in temperature of the water from the time it leaves
the nozzle until it reaches the ground ; tw is the mean water temperature,
degrees F ; ta is the air temperature, degrees F ; Pw is the vapor pressure
of water at temperature tw, inches of mercury ; Pa is the pressure of the
water vapor in the air, inches of mercury • and B is the barometric pres-
Bul. 670] Irrigation by Sprinkling 113
sure, inches of mercury. Evaluating the constants, letting C = 0.555,
r = 585, and B = 30, the expression becomes
E = 0.095 At [~- J>W~Fa , rl . (27)
\PW -Pa- 0.011 (ta-tw)j
The expression within the brackets varies from less than 1.0, when
the temperature of the water exceeds the air temperature, to infinity
when the water temperature is the same as the wet-bulb temperature ;
and from minus infinity to zero as the water temperature decreases to
the dew point.
Equation 27 fails to take into consideration the very small drops of
water which are completely evaporated or blown away by the wind,
and which do not contribute to the final temperature of the water as it
reaches the ground. A study of the distribution of size of drops, however,
indicates that only a very small part of the water discharged is in the
form of tiny drops that are lost, and it is believed that this would not
cause an appreciable error in the determination of the evaporation loss
from the spray.
Several tests have been made to determine the temperature drop of
the water between the sprinkler and the ground. The water was caught
in a thermos bottle with a large funnel. To obtain a large quantity of
water quickly and to avoid errors caused by changes in the temperature
of the water after it is caught, the rotating sprinklers are stopped during
the catch. For some of these tests, in order to increase the evaporation
loss and to more nearly approach field conditions, the water was warmed
several degrees above the normal temperature of the well water which
was about 65° F. This was accomplished by installing two electric heaters
in the pipe line supplying water to the sprinkler. The temperatures were
read with a thermometer graduated to 0.2° F. Five tests on rotating
sprinklers with initial water temperatures of 69.5° to 84°, and with air
temperatures ranging from 75° to 101°, showed decreases in water tem-
perature from 1° to 7° corresponding to evaporation losses of 0.23 to
0.81 per cent. Another test on a small spray nozzle, with an initial water
temperature of 98.7° and an air temperature of 100.5°, resulted in a
temperature drop of 20.7°, corresponding to a loss of about 2 per cent.
From these measurements of the temperature change of the water as
it passes through the air, one must conclude that the evaporation loss
from the spray is negligible in comparison with subsequent losses from
the wet soil and vegetation. Even when one considers the possible effect
of solar radiation, and the increase in vapor pressure of the water when
in the form of small drops, one cannot account for evaporation losses
from the spray of more than 2 per cent of the amount applied.
114 University of California — Experiment Station
EVAPORATION LOSSES FROM WET SURFACES
Evaporation from soils and other wet surfaces, during and following
an application of water, may be appreciable. This loss is relatively more
important in connection with irrigation by sprinkling than with irriga-
tion by other methods because (1) lighter applications are generally
made by sprinkling than by surface irrigation, so that a larger percent-
age of the water applied may be lost by subsequent evaporation; and (2)
the cost of applying water by sprinkling is generally higher, so that the
evaporation loss represents a greater economic loss.
Evaporation Losses from Moist Soils. — The evaporation loss from
moist soils has long been a subject of discussion and experiment by those
interested in irrigation. Fortier and Beckett20 conducted experiments
to determine the loss from undisturbed and cultivated soils at Davis
after an irrigation. Veihmeyer21 made similar studies at Mountain View.
Briefly, these experiments indicate that 1 to 2 inches of water evaporates
from the soil within 3 to 4 weeks after an irrigation, more than half of
which occurs during the first five days. This loss is not prevented by
cultivation, partly because most of it occurs before the soil is dry enough
to work. Practically all of the evaporation loss is from the first foot of
soil, and most of it from the first 4 inches. The rate of loss by evaporation
after the first week is negligible in comparison with the rate of extraction
of water by plants.
The actual loss by direct evaporation from a cropped soil is probably
considerably less than from a bare soil because the ground is shaded,
partially at least, by the crop, and also because the growing crop rapidly
reduces the moisture content of the soil by transpiration, which lessens
the opportunity for evaporation to occur.
According to other experiments, the rate of evaporation from satu-
rated soils is about the same as from a free water surface, or about 0.3
inch per day in the Sacramento Valley during the summer. With a water
table 6 inches below the surface, the evaporation loss averaged about
0.21 inch per day during the summer; but this decreased to about 0.06
inch per day with the water table 2 feet below the surface.
When relatively small amounts of water are applied to exposed soils
at frequent intervals by sprinkling, a common practice in the spring,
much of the water may be lost by evaporation. In some instances appli-
cations of about 1 inch are made at weekly intervals to aid the germina-
tion and starting of a crop and to prevent the drying out and crusting
20 Fortier, Samuel, and S. H. Beckett. Evaporation from irrigated soils. U. S. Dept.
Agr. Office of Experiment Stations Bui. 248:1-77. 1912.
21 Veihmeyer, F. J. Some factors affecting irrigation of deciduous orchards. Hil-
gardia 2(6): 125-291. 1927.
Bul. 670] Irrigation by Sprinkling 115
of the surface soil at this time. Most of this water may be lost directly
from the soil by evaporation. When, on the other hand, a growing crop
completely covers and shades the soil, and when the applications are
sufficient to give a penetration of 3 to 6 feet, only a relatively small per-
centage of that applied may be lost by direct evaporation.
Bate of Evaporation from Free Water Surfaces. — The evaporation
from free water surfaces is measured at a number of Weather Bureau
stations, and at many other places in California and elsewhere. The
evaporation as measured at these stations has been found to depend
largely upon the type of evaporation pan used, and upon its exposure.
For this reason various organizations, such as the Weather Bureau, have
standardized their evaporation equipment to facilitate comparisons.
Tests have also been made to determine the coefficients by which the
evaporation from various types of pans must be multiplied to obtain
the equivalent evaporation from a lake surface. For the standard Class
A Weather Bureau pan the coefficient most generally used is 0.70.22 A
Class A Weather Bureau evaporation station has been maintained at
Davis since 1926. Besides the regular equipment, a recording evapori-
meter is used so that instantaneous rates of evaporation can be obtained.
The average daily evaporation at Davis for a fourteen-year period was
as follows: May, 0.27 inch; June, 0.31; July, 0.35; August, 0.32; and
September, 0.24. According to evaporimeter records, the maximum
hourly rate sometimes exceeds 0.05 inch and averages about 0.04 during
the afternoon.
Since water is sometimes applied with sprinklers at rates as low as
0.10 inch per hour, an appreciable evaporation loss may occur during
and immediately after an application. Even with application rates of
0.25 to 0.50 inch per hour, more than 10 per cent of the water may evap-
orate as it is applied during the afternoon. The evaporation loss at night,
however, is usually very low.
Interception and Subsequent Evaporation of Water from Plants. —
When crops are sprinkled, part of the water is intercepted by the foliage
and later evaporated without reaching the soil. Studies of rainfall in-
terception by various investigators indicate that an appreciable amount
may be caught by trees and other plants, especially when the rain occurs
in small storms. The determination of rainfall reaching the soil under
a vegetative cover is difficult, and results obtained are not always con-
sistent. Furthermore, the interception is generally reported in per cent
of rainfall, and therefore depends upon the intensity and duration of
22 American Society of Civil Engineers. Evaporation from water surfaces, a sym-
posium. Amer. Soc. Civ. Engin. Trans. 99:671-747. 1934.
116 University of California — Experiment Station
the storm. Clark23 made determinations of the maximum interception
capacity of many plants. From his data it appears that few crops can
retain 0.1 inch of water, although his attempts to measure interception
by catching water in pans under the vegetation indicate much larger
losses.
Interception of rainfall and that for water applied with sprinklers
should differ principally in the amount that evaporates during the ap-
plication. The evaporation rate while sprinkling may be high, whereas
during a rain evaporation rates are generally very low. The presence
of water on the foliage should temporarily reduce the rate of evaporation
from the soil and the rate of transpiration from the leaves.
DESIGN AND OPERATION OF SPRINKLER SYSTEMS
Methods of Operation, — With respect to the method of operation, por-
table sprinkler systems may be divided into two general classes : first,
those in which the rate of application is fairly high and the sprinkler
lines are moved frequently; second, those in which the rate is relatively
low and the moves are rather infrequent. Most systems with portable
pumping plants fall into the first class. The capacity of the system is
generally limited by the power available for operating the pump, while
the length of portable pipe that can be effectively used is governed by
the dimensions of the field and the general layout of the system. The
sprinklers selected are of a capacity that will effectively utilize the pump-
ing plant. The rate at which the water is applied receives little considera-
tion ; often it is too high, and water accumulates on the ground surface
before adequate penetration is obtained. The result, usually, is that light
frequent applications are made where heavier and less frequent applica-
tions would give better results and be more economical. Frequent moves
mean continuous employment of labor together with a relatively low
operating efficiency because of the larger proportion of time lost. This
is especially true with the single-line arrangement. Most of these systems
operate continuously day and night, the lines being moved every 2 to 6
hours. Moving pipe at night is objectionable because it takes a crew con-
siderably longer to make moves at night than during the day, and because
working conditions at night are disagreeable.
Under the second method of operation, the water is applied at such a
rate that the moves can be made at convenient times each day. The ideal
arrangement is to move the lines twice a day — morning and evening so
that both moves are made in daylight. From 3 to 8 inches of water is
generally required to wet dry soils to the depth from which moisture is
23 Clark, O. R. Interception of rainfall by prairie grasses, weeds, and certain crop
plants. Ecological Monographs 10:243-77. Apr., 1940.
Bul. 670] Irrigation by Sprinkling 117
extracted by most crops. Applications of these amounts must be made
fairly slowly so that the soil can absorb the water without runoff. Under
some conditions it is necessary to extend the period of application to 24
hours to avoid runoff.
When sprinkler systems are operated on this basis, continuous atten-
tion may not be required. The operating cost will then be lower, since 60
to 80 per cent of the actual expense of operating portable systems is for
labor. If this help can be employed only during the time required for
moving pipe, or used effectively elsewhere during the time when they are
not needed, the saving will be appreciable. There are many possible
schemes for operating a system in this manner.
When a portable pumping plant is used, continuous attention may be
necessary to prevent damage if the pump loses its prime or if any other
emergencies arise. When moves are made only twice daily, it may be
practicable to employ one man for the day shift and one man for the
night shift, having the shifts overlap at the time of moving. In this case,
each shift would be 12 hours plus the time required to move the system.
Long shifts are feasible because during most of the time no work is re-
quired and the operator can rest.
Where adequate applications can be made in about 8 hours, three
moves a day may be desirable. With one move at dawn, another at noon,
and a third just before dark, three fairly equal periods of operation can
be obtained during the summer months. Two men working alternate
shifts overlapping the moving period could handle a system on this
schedule with less physical effort than under the conditions now common.
Three men working overlapping shifts of about 9 hours may, however,
be more desirable for such a schedule.
For small sprinkling systems operated by the owner, a 12-hour set is
especially advantageous ; it permits the farmer to carry on his irrigation
work continuously, and yet take care of other farm operations. This
method of operation requires a system that is free from troubles ; sprin-
klers sometimes clog or stop rotating, or the pump may lose its prime.
Usually it is more feasible when pressure supply lines and stationary
electric-driven pumping plants are used. At present this method is being
practiced to a much greater extent with orchard systems than with other
types.
Capacity of Sprinkler Systems. — In planning any sprinkler system
one must first consider those factors that influence the selection of the
size of the system. In general, the lowest initial cost results when a system
is planned for continuous operation with a capacity that will just satisfy
crop requirements. This may or may not be the most economical system
to operate. Sprinkler systems can be designed to use effectively a smaller
118 University of California — Experiment Station
flow than is desirable for any other method of operation — a feature ad-
vantageous to the operator of a small farm or orchard, since he need not
invest in an irrigation system with a capacity several times larger than
necessary for his acreage.
Whether or not a small sprinkler system, operated more or less con-
tinuously, will be more economical than a larger system, operated only
a few days for each irrigation, depends primarily upon the method of
operation. If it requires continuous attention, a small system may not
be economical because the labor cost of operating it may be very high —
all out of proportion to the cost of the water or the cost of power for
pumping. Where continuous attention is not necessary, and especially
where the attention required can be given at convenient times, a small
system operating continuously will also be the most economical to oper-
ate. Such systems are especially desirable for orchards and pastures.
For large field-crop portable systems with portable pumping plants, one
may find it difficult to arrange the layout for slow rates of application
that will permit 12-hour or even 8-hour periods of operation.
Sprinkler systems designed for continuous operation must have suffi-
cient capacity to meet the peak requirements during the critical part of
the year. For orchards and perennial crops the maximum water require-
ment occurs during June, July, and August. For some annual crops it
may occur earlier or later in the season. Crop transpiration depends
largely upon the climate — sunshine, temperature, and humidity. For
interior valley conditions, crops that completely cover the ground —
alfalfa, sugar beets, and the like — will transpire water at a maximum
rate of about 6 to 8 inches per month during the period June to August.
Mature deciduous trees transpire about the same amount. Citrus require-
ments are a little lower. For some coastal areas, applications of 2 to 4
inches per month are adequate.
One acre-inch of water in 30 days is equivalent to 0.628 gallon per
minute continuous flow. To supply 8 inches in a month, therefore, re-
quires a continuous flow of about 5 gallons per minute per acre. For
valley conditions, this is approximately the minimum flow that will com-
pletely satisfy crop requirements. For systems designed to be operated
only during the day, a flow of about 10 gallons per minute per acre will
be required. Where additional time must be allowed for cultural opera-
tions, systems with still greater capacity must be provided. Where con-
ditions are such that it is economical to provide a complete irrigation in
a short period, a still larger flow may be desirable. A convenient relation
to remember is that the required flow
Q = ^ (28)
Bul. 670] Irrigation by Sprinkling 119
where Q is the flow in gallons per minute ; d is the average depth of water
applied in inches ; A is the area in acres, and T is the time in hours. For
example, to apply a 4-inch application on a 10-acre tract in 48 hours
requires
. 450X4X10 o^c __
Q = — = 375 gallons per mmute.
Bate of Application. — Besides the capacity of the system, one must
consider the rate of application. This may be governed by the soil con-
dition (the rate at which the soil will absorb the water) or by the method
of operation (the period of time in which one wishes to apply a given
amount of water) .
Coarse- textured soils (sandy and gravelly soils) generally absorb
water rapidly, usually at any normal rate at which it might be applied
with a sprinkler system. Fine-textured soils (fine silts and clays) absorb
water more slowly. The rate of absorption decreases with time. Some clay
adobe-type soils absorb water very rapidly when dry because of the
granular structure and large number of cracks that form when the soil
dries. For example, when dry, such a soil might absorb the water at a rate
of several inches per hour for a few minutes ; but the rate of absorption
might decrease rapidly, perhaps becoming a small fraction of an inch
per hour after several hours of application.
The maximum rate at which the water should be applied will depend
upon the amount of each application. When only 1 or 2 inches of water
is desired, high rates of application may be used ; but if 4 to 6 inches or
more is to be applied, much slower rates are required. A knowledge of
rainfall rates and of the ability of soil to absorb prolonged rains may be
used as a basis for selecting suitable rates for sprinkler application.
The rate at which soils absorb water also depends somewhat upon how
the water is applied. Large drops resulting from low pressures tend to
puddle and seal the surface of the soil much sooner than a fine atomized
spray. The condition of the soil surface is also important. A cultivated
soil left somewhat cloddy will absorb the water more rapidly than one
that has been pulverized to a fine dust. A hard compacted or crusted soil
will generally absorb water slowly. The presence of organic matter on
the surface appreciably increases the rate of absorption for some soils.
Rates of application varying from about 0.1 to more than 1 inch per hour
are practicable with sprinkler systems.
Depth of Application. — Soils act as a reservoir for water, retaining
in it the pore spaces between the soil particles until it is used by plants
or lost by evaporation. When a soil is irrigated, the water penetrates
rather rapidly until the moisture content is reduced by drainage to a
120 University of California — Experiment Station
certain percentage, called the field capacity, after which further move-
ment and penetration into dry soil becomes very slow. For practical pur-
poses, the field capacity may be considered as the amount of moisture a
soil will retain against the downward force of gravity, or the upper limit
of the available-moisture range. It varies greatly with soil texture, and
to lesser extent with other factors. In general, fine-textured soils have a
higher field capacity than coarse-textured soils ; clay loams and clays
range from about 20 to 30 per cent, whereas fine sands and sandy loams
vary from about 8 to 15 per cent. In terms of quantity, the amount of
water held by a soil at field capacity varies from about 1 inch per foot
depth of soil for sandy loam to 4 inches for some clays.
Not all the water held by a soil is available for plant use. The lower
limit of the readily available moisture range is called the permanent
wilting percentage. At this moisture percentage plants wilt and do not
revive until water is added to the soil. Generally plants do not die when
they wilt, but their growth practically ceases ; in some instances, they
shed their leaves. The permanent wilting percentage is the same for all
kinds of plants grown in the same soil, although it varies widely with
different soils. Sometimes, however, shallow-rooted plants wilt before
other deeper-rooted plants growing side by side. Plants differ in their
ability to withstand long periods in dry soils ; some die, others go into a
dormant state.
The permanent wilting percentage is influenced primarily by soil
texture ; for many soils, it is approximately half that of the field capacity.
This rule, however, cannot be applied generally because some soils show
wide departures from it. A typical Aiken clay loam, for example, with
a field capacity of about 30 per cent has a wilting percentage of 21 per
cent. In contrast, a typical Fresno fine sandy loam has a field capacity
of about 10 per cent and a permanent wilting percentage of about 3.
The Aiken clay loam holds three times as much water as the Fresno fine
sandy loam; but that available to plants is nearly the same.
In addition to the ability of the soil to hold water, the depth of applica-
tion is influenced by the rooting depth of the crop grown. This varies
with different crops, and also depends upon the soil and upon the pres-
ence of hardpan or a water table near the surface. For many field crops
growing in deep uniform soils, roots are active to much greater depths
than is commonly supposed. Judging from recent investigation at
Davis24, sugar beets extract moisture from a depth of 6 feet ; tomatoes
from about 7 feet. Sugar beets showed little evidence of need for water
until the first 4 feet of soil was dried to the permanent wilting percentage.
In contrast, potatoes growing in an Aiken loam indicated a need for
2i Doneen, L. D. Studies in the irrigation of sugar beets. Pacific Eural Press 131
(8):307. 1941.
Bul. 670] Irrigation by Sprinkling 121
water when the first foot of soil was dry, though there was still available
water in the second foot.25 This difference is attributed partly to the soil,
as other crops growing in this soil require frequent irrigations. Studies
at Shaf ter26 showed that cotton extracts moisture from a depth of 6 feet.
Alfalfa roots extend well below 6 feet.27,28 Generally, deciduous trees
are deep-rooted. Prunes29 extract moisture from below 9 feet, and walnut
roots30 are active to a depth of 12 feet. Although the roots of citrus31 ex-
tend below 4 feet, most of the extraction is from the first 2 feet.
In general, it is considered good irrigation practice to wet the soil to
the depth of the rooting zone (that depth from which water is extracted
by plants) each time it is irrigated. Sprinkling is practiced on many
shallow soils underlaid with rock, hardpan, gravel, or a high water table.
Care must then be exercised ; if too much water is applied, the soil may
become waterlogged, or an appreciable amount may be lost by deep
percolation.
Sometimes a shallow sandy loam surface soil is underlaid with a very
heavy subsoil. Because of the increased root activity in the surface soil,
and also because of its lower water-yielding capacity, the moisture is
extracted from the surface soil sooner than from the subsoil, and rather
frequent irrigations are required to maintain available moisture in it.
"When excessive applications are made, the excess water drains into the
already wet subsoil, and may actually keep it in a more or less saturated
condition. Roots tend to die out in the wet subsoil and become concen-
trated near the surface. Sprinkling has been found especially beneficial
in overcoming this condition in citrus orchards. Light, frequent appli-
cations maintain available moisture in the surface soil without contrib-
uting water to the subsoil. Over a period of time this permits roots to
develop to a greater depth. Greatly improved tree conditions have been
noted in some instances.
Under different soil and crop conditions, applications varying from
1 to 6 inches or more might be desirable. Sprinkling is most extensively
25 Edlef sen, N. E. Effect of soil moisture characteristics on irrigation requirements.
Agr. Engin. 18:247-50. 1937.
26 Beckett, S. H., and Carroll F. Dunshee. Water requirements of cotton on sandy
loam soils in southern San Joaquin Valley. California Agr. Exp. Sta. Bul. 537:1-48.
1932. (Out of print.)
27 Beckett, S. H., and M. E. Huberty. Irrigation investigations with field crops at
Davis, and at Delhi, California 1909-1925. California Agr. Exp. Sta. Bul. 450:1-24.
1928. (Out of print.)
28 Unpublished data, experiments in progress.
29 Hendrickson, A. H., and F. J. Veihmeyer. Irrigation experiments with prunes.
California Agr. Exp. Sta. Bul. 573 : 1-44. 1934.
80 Veihmeyer, F. J., and A. H. Hendrickson. Soil moisture as an indication of root
distribution in deciduous orchards. Plant Physiol. 13(1) : 169-77. 1938.
31 Beckett, S. H., Harry F. Blaney, and Colin A. Taylor. Irrigation water require-
ment studies of citrus and avocado trees in San Diego County, California, 1926 and
1927. California Agr. Exp. Sta. Bul. 489:1-51. 1930.
122 University of California — Experiment Station
practiced under conditions requiring light frequent applications, where
it has a relatively greater advantage over other irrigation methods. Ap-
plications of 2 to 4 inches are most common, and satisfy requirements
in many places.
To summarize : a sprinkler system should be planned with adequate
capacity for crop requirements, and it should be capable of applying
proper amounts at such rates that the soil can absorb the water without
runoff.
Sprinkler Capacity. — Having decided upon the general method of
operation, the required capacity of the system, and the suitable rates of
application, one comes to the more technical phases of the design prob-
lem. The general layout of the system and the spacing of sprinklers
must be considered. To determine the proper spacing for best distribu-
tion of water one must understand sprinkler performance. The capacity
of the sprinklers may be governed by the dimensions of the field and the
total capacity of the system, or by the rate at which the water can be
applied. For large portable systems, with portable pumping plants, the
capacity of the system is usually limited by the power available for
pumping, and the capacity of the sprinklers by the number that can
effectively be used, which depends upon spacing and field dimensions.
Sometimes the capacity of a system is governed by the sprinkler dis-
charge required to make a given application in a certain time interval,
and by the number of sprinklers for most effective arrangement.
The required sprinkler discharge, to apply a given quantity of water
in a certain period, can be conveniently calculated from the following
expression :
q = ^r (29)
where q is the discharge of each sprinkler in gallons per minute ; d is the
average depth of application, inches; #! is the sprinkler spacings along
the line ; $2 is the spacing between sprinkler lines ; and T is the period
of operation in hours.
Consider, for example, a typical case — an 80-acre field to be irrigated
by a portable system supplied by a portable pump from a field ditch
running lengthwise through the middle of the field. From the standpoint
of crop requirements and method of operation a total capacity of 5 gal-
lons per minute per acre or 400 gallons per minute will suffice. The net
field dimensions are 1,290 by 2,610 feet. Six hundred feet of pipe will
reach from the pump near enough to the edges of the field for the last
sprinkler to cover. Fifteen sprinklers 40 feet apart, twenty sprinklers
30 feet apart, or thirty sprinklers 20 feet apart might be used on a 600-
foot line; or twice as many on 1,200 feet of pipe.
Bul. 670] Irrigation by Sprinkling 123
Suppose a 12-hour operating schedule is preferred, and a 4-inch appli-
cation is considered adequate. Because of wind conditions, the lines can
be moved a maximum of 60 feet. To make the number of sets come out
even, the lines should actually be moved only 59 feet. To apply 4 inches
of water in about 11 hours, allowing for moves, with #1=40, and $2=59
would require a sprinkler discharge of
4 X 40 X 59
q = = 9 gallons per minute .
* 96X11
With a 40-foot spacing on the line, an effective arrangement necessi-
tates either fifteen sprinklers on a single line, or thirty on a split line.
With thirty sprinklers, the total capacity would be only 270 gallons per
minute, which is not adequate. A minimum capacity of zQV-= 13.3 gal-
oU
Ions per minute is considered necessary. With three moves per 24 hours,
allowing 7 hours' net operating time between moves, the sprinkler capac-
ity would be
4 X 40 X 59
q = — = 14 gallons per minute.
96X7 * P
Thirty sprinklers would have a total capacity of 420 gallons per min-
ute. With three moves, or 177 feet of land covered per day, a total of
2590 __ 246 days, or 44 settings would be required to cover the field.
177
This would allow for two 4- inch irrigations per month.
Size of Pipe. — The size of pipe required in a sprinkler system depends
upon several factors. In general, the size should be such that the friction
loss is reasonable. A high friction loss in the pipe lines makes it difficult
or impossible to distribute the water uniformly over the area. Where
water is supplied under pressure, pipe sizes must be ample to carry the
required flow and leave a sufficient pressure for satisfactory sprinkler
performance. Where pumping is required, the problem is fundamentally
one of determining what sizes result in the lowest annual cost. Small
pipe is less in first cost but it increases the pumping head and cost of
pumping. There are two methods of approaching this problem. One is
to set down, in the form of an integral equation, all items entering into
the cost and differentiate to determine what pipe size gives minimum
cost.82 The other is to estimate the total annual cost for different pipe
sizes that appear practicable, and in that way to determine the most
economical size. Both necessitate assumptions regarding cost and life
of pipe, that are at best only approximations.
32 This method of attack is discussed in : Gladding, R. D. Special design problems
in distribution systems for sprinkler irrigation. A paper presented at the Pacific
Coast Section Meeting, American Society of Agricultural Engineers, Corvallis,
Oregon, February 16, 1940. 22 p. (Mimeo.)
124 University of California — Experiment Station
Friction losses in pipe of different kinds are discussed in detail in
the section "Hydraulics of Sprinkler Systems." When using friction-loss
tables or diagrams for the purpose of selecting pipe sizes, one may
well use conservative friction factors, or coefficients, or add a certain
amount — about 10 per cent — to the required flow as a factor of safety.
The same principles apply to the design of stationary systems. The uni-
formity of distribution is then especially important, Careful considera-
tion must be given to the arrangement and spacing of sprinklers, and to
the type of sprinkler pattern. Minimum cost will result with low rates
of application which permit smallest pipe sizes. Although labor costs
are lower, stationary systems should also be designed for convenient
operation.
Many other problems enter into the design of sprinkler systems. Some
of these have been discussed in connection with the descriptions of the
various systems. Although the design of a sprinkler system is essentially
an engineering problem, it is also one that requires a knowledge of
agriculture.
ACKNOWLEDGMENTS
The author wishes to acknowledge the cooperation of the various
sprinkler-equipment manufacturers who furnished pipe and sprinklers
for tests reported in this bulletin. He also thanks Professors F. J. Veih-
meyer, B. D. Moses, M. R. Huberty, A. F. Pillsbury, and Mr. J. B.
Brown for reviewing the manuscript and offering many helpful sug-
gestions.
16im-ll,'42(324)
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